Patterning process, resist composition, polymer, and monomer

ABSTRACT

A negative pattern is formed by applying a resist composition onto a substrate, prebaking, exposing to high-energy radiation, PEB, and developing the exposed resist film in an organic solvent developer to dissolve the unexposed region of resist film. The resist composition comprising a polymer adapted to form a lactone ring under the action of an acid so that the polymer may reduce its solubility in an organic solvent displays a high dissolution contrast. A fine hole or trench pattern can be formed therefrom.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2013-005982 filed in Japan on Jan. 17, 2013,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a radiation-sensitive chemically amplifiedresist composition which is advantageously used in the micropatterninglithography using a variety of radiations including deep-UV, typicallyKrF and ArF excimer lasers, extreme ultraviolet (EUV), x-ray, typicallysynchrotron radiation, and charged particle beam, typically electronbeam, and a pattern forming process using the same. More particularly,it relates to a pattern forming process involving exposure of resistfilm, modification of base resin by chemical reaction with the aid ofacid and heat, and development in an organic solvent to form a negativetone pattern in which the unexposed region is dissolved and the exposedregion is not dissolved; a resist composition for use in the process; apolymer; and a monomer.

BACKGROUND ART

In the recent drive for higher integration and operating speeds in LSIdevices, the pattern rule is made drastically finer. Thephotolithography which is currently on widespread use in the art isapproaching the essential limit of resolution determined by thewavelength of a light source. As the light source used in thelithography for resist pattern formation, g-line (436 nm) or i-line (365nm) from a mercury lamp was widely used in 1980's. Reducing thewavelength of exposure light was believed effective as the means forfurther reducing the feature size. For the mass production process of 64MB dynamic random access memories (DRAM, processing feature size 0.25 μmor less) in 1990's and later ones, the exposure light source of i-line(365 nm) was replaced by a KrF excimer laser having a shorter wavelengthof 248 nm. However, for the fabrication of DRAM with a degree ofintegration of 256 MB and 1 GB or more requiring a finer patterningtechnology (processing feature size 0.2 μm or less), a shorterwavelength light source was required. Over a decade, photolithographyusing ArF excimer laser light (193 nm) has been under activeinvestigation. It was expected at the initial that the ArF lithographywould be applied to the fabrication of 180-nm node devices. However, theKrF excimer lithography survived to the mass-scale fabrication of 130-nmnode devices. So, the full application of ArF lithography started fromthe 90-nm node. The ArF lithography combined with a lens having anincreased numerical aperture (NA) of 0.9 is considered to comply with65-nm node devices. For the next 45-nm node devices which required anadvancement to reduce the wavelength of exposure light, the F₂lithography of 157 nm wavelength became a candidate. However, for thereasons that the projection lens uses a large amount of expensive CaF₂single crystal, the scanner thus becomes expensive, hard pellicles areintroduced due to the extremely low durability of soft pellicles, theoptical system must be accordingly altered, and the etch resistance ofresist is low; the development of F₂ lithography was stopped andinstead, the ArF immersion lithography was introduced.

In the ArF immersion lithography, the space between the projection lensand the wafer is filled with water having a refractive index of 1.44.The partial fill system is compliant with high-speed scanning and whencombined with a lens having a NA of 1.3, enables mass production of45-nm node devices.

One candidate for the 32-nm node lithography is lithography usingextreme ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUVlithography has many accumulative problems to be overcome, includingincreased laser output, increased sensitivity, increased resolution andminimized edge roughness (LER, LWR) of resist film, defect-free MoSilaminate mask, reduced aberration of reflection mirror, and the like.

Another candidate for the 32-nm node lithography is high refractiveindex liquid immersion lithography. The development of this technologywas stopped because LUAG, a high refractive index lens candidate had alow transmittance and the refractive index of liquid did not reach thegoal of 1.8.

The organic solvent development to form a negative pattern is atraditional technique. A resist composition comprising cyclized rubberis developed using an alkene such as xylene as the developer. An earlychemically amplified resist composition comprisingpoly(tert-butoxycarbonyloxystyrene) is developed with anisole as thedeveloper to form a negative pattern.

Recently a highlight is put on the organic solvent development again. Itwould be desirable if a very fine hole pattern, which is not achievablewith the positive tone, is resolvable through negative tone exposure. Tothis end, a positive resist composition featuring a high resolution issubjected to organic solvent development to form a negative pattern. Anattempt to double a resolution by combining two developments, alkalinedevelopment and organic solvent development is under study.

As the ArF resist composition for negative tone development with organicsolvent, positive ArF resist compositions of the prior art design may beused. Such pattern forming processes are described in Patent Documents 1to 3. These patent documents disclose resist compositions for organicsolvent development comprising a copolymer of hydroxyadamantanemethacrylate, a copolymer of norbornane lactone methacrylate, and acopolymer of methacrylate having acidic groups including carboxyl,sulfo, phenol and thiol groups substituted with two or more acid labilegroups, and pattern forming processes using the same.

Further, Patent Document 4 discloses a process for forming a patternthrough organic solvent development in which a protective film isapplied onto a resist film.

Patent Document 5 discloses a topcoatless process for forming a patternthrough organic solvent development in which an additive is added to aresist composition so that the additive may segregate at the resist filmsurface after spin coating to provide the surface with improved waterrepellency.

CITATION LIST

Patent Document 1: JP-A 2008-281974

Patent Document 2: JP-A 2008-281975

Patent Document 3: JP 4554665

Patent Document 4: JP-A 2008-309878

Patent Document 5: JP-A 2008-309879

DISCLOSURE OF INVENTION

As compared with the positive resist system which becomes dissolvable inalkaline developer as a result of acidic carboxyl or analogous groupsgenerating through deprotection reaction, the organic solventdevelopment provides a low dissolution contrast. The alkaline developerprovides an alkaline dissolution rate that differs by a factor of 1,000or more between the unexposed and exposed regions whereas the organicsolvent development provides a dissolution rate difference of only about10 times. While Patent Documents 1 to 5 describe conventionalphotoresist compositions of the alkaline aqueous solution developmenttype, there is a demand for a novel material which can offer asignificant dissolution contrast upon organic solvent development. Thatis, a strong demand exists for a material capable of providing theunexposed region of promoted dissolution and the exposed region ofretarded dissolution in an organic solvent developer.

An object of the invention is to provide a pattern forming processinvolving organic solvent development for forming a negative tonepattern having a significant dissolution contrast between the unexposedregion of promoted dissolution and the exposed region of retardeddissolution. Another object is to provide a resist composition for usein the process; a polymer; and a monomer.

The inventors have found that the dissolution contrast during organicsolvent development is improved using a polymer adapted to form alactone ring under the action of an acid so that the polymer may reduceits solubility in an organic solvent.

In one aspect, the invention provides a pattern forming processcomprising the steps of applying a resist composition comprising apolymer adapted to form a lactone ring under the action of an acid sothat the polymer may reduce its solubility in an organic solvent, and anoptional acid generator onto a substrate, prebaking the composition toform a resist film, exposing the resist film to high-energy radiation,baking, and developing the exposed film in an organic solvent-baseddeveloper to form a negative pattern wherein the unexposed region ofresist film is dissolved away and the exposed region of resist film isnot dissolved, the polymer comprising recurring units (a1) of thegeneral formula (1).

Herein R¹ is hydrogen or methyl, R² is hydrogen or a straight, branchedor cyclic monovalent hydrocarbon group of 1 to 20 carbon atoms in whicha constituent —CH₂— may be substituted by —O— or —C(═O)—, R³ is an acidlabile group, R⁴ is hydrogen or a straight, branched or cyclicmonovalent hydrocarbon group of 1 to 20 carbon atoms in which aconstituent —CH₂— may be substituted by —O— or —C(═O)—, and a1 is anumber in the range: 0<a1<1.0.

In one preferred embodiment, the polymer further comprises recurringunits (a2) of the general formula (2) and recurring units of at leastone type selected from recurring units (b1) to (b4) represented by thegeneral formula (3).

Herein R¹⁰ is hydrogen or methyl, R¹¹ is an acid labile group differentfrom the acid labile group R³ in formula (1), X³ is a single bond,phenylene, naphthylene, or —C(═O)—O—R¹²—, R¹² is a straight, branched orcyclic C₁-C₁₀ alkylene group which may have an ether moiety, estermoiety, lactone ring or hydroxyl moiety, or phenylene or naphthylenegroup, and 0≦a2<1.0.

Herein R¹³ and R¹⁶ each are hydrogen or methyl, R¹⁴ is a straight,branched or cyclic, di- to pentavalent aliphatic hydrocarbon group of 1to 16 carbon atoms which may have an ether or ester moiety, R¹⁵ and R¹⁷each are an acid labile group, R¹⁸ to R²¹ and R²² to R²⁵ are eachindependently hydrogen, cyano, a straight, branched or cyclic C₂-C₆alkyl group, alkoxycarbonyl, or a group having an ether moiety orlactone ring, at least one of R¹⁸ to R²¹ and R²² to R²⁵ has a hydroxylgroup substituted with an acid labile group, m is an integer of 1 to 4,n is 0 or 1, b1, b2, b3 and b4 are numbers in the range: 0≦b1<1.0,0≦b2<1.0, 0≦b3<1.0, 0≦b4<1.0, 0≦b1+b2+b3+b4<1.0, and0<a2+b1+b2+b3+b4<1.0.

In one preferred embodiment, the polymer further comprises recurringunits derived from a monomer having an adhesive group selected fromamong hydroxyl, cyano, carbonyl, ester, ether, lactone ring, carboxyl,carboxylic anhydride, sulfonic acid ester, disulfone, and carbonate.

In one preferred embodiment, the polymer further comprises recurringunits (d1), (d2) or (d3) represented by the following general formula:

wherein R⁰²⁰, R⁰²⁴, and R⁰²⁸ each are hydrogen or methyl; R⁰²¹ is asingle bond, phenylene, —O—R⁰³³—, or —C(═O)—Y—R⁰³³—, wherein Y is oxygenor NH and R⁰³³ is a straight, branched or cyclic C₁-C₆ alkylene group,alkenylene group or phenylene group, which may contain a carbonyl(—CO—), ester (—COO—), ether (—O—), or hydroxyl moiety; R⁰²², R⁰²³,R⁰²⁵, R⁰²⁶, R⁰²⁷, R⁰²⁹, R⁰³⁰, and R⁰³¹ are each independently astraight, branched or cyclic C₁-C₁₂ alkyl group which may contain acarbonyl, ester or ether moiety, a C₆-C₁₂ aryl group, a C₇-C₂₀ aralkylgroup, or a thiophenyl group; Z¹ is a single bond, methylene, ethylene,phenylene, fluorinated phenylene, —O—R⁰³²—, or —C(═O)—Z²—R⁰³²—, whereinZ² is oxygen or NH, and R⁰³² is a straight, branched or cyclic C₁-C₆alkylene group, alkenylene group or phenylene group, which may contain acarbonyl, ester, ether or hydroxyl moiety; M⁻ is a non-nucleophiliccounter ion; 0≦d1≦0.3, 0≦d2≦0.3, 0≦d3≦0.3, 0<d1+d2+d3≦0.3.

In one preferred embodiment, the developer comprises at least oneorganic solvent selected from among 2-octanone, 2-nonanone, 2-heptanone,3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone,methylcyclohexanone, acetophenone, methylacetophenone, propyl acetate,butyl acetate, isobutyl acetate, amyl acetate, isoamyl acetate, butenylacetate, propyl formate, butyl formate, isobutyl formate, amyl formate,isoamyl formate, methyl valerate, methyl pentenoate, methyl crotonate,ethyl crotonate, methyl propionate, ethyl propionate, ethyl3-ethoxypropionate, methyl lactate, ethyl lactate, propyl lactate, butyllactate, isobutyl lactate, amyl lactate, isoamyl lactate, methyl2-hydroxyisobutyrate, ethyl 2-hydroxyisobutyrate, methyl benzoate, ethylbenzoate, phenyl acetate, benzyl acetate, methyl phenylacetate, benzylformate, phenylethyl formate, methyl 3-phenylpropionate, benzylpropionate, ethyl phenylacetate, and 2-phenylethyl acetate.

In one preferred embodiment, the step of exposing the resist film tohigh-energy radiation includes KrF excimer laser lithography of 248 nmwavelength, ArF excimer laser lithography of 193 nm wavelength, EUVlithography of 13.5 nm wavelength or EB lithography.

Another embodiment is a pattern forming process comprising the steps ofapplying a resist composition comprising a polymer comprising recurringunits of formula (1), an optional acid generator, and an organic solventonto a substrate, prebaking the composition to form a resist film,forming a protective film thereon, exposing the resist film tohigh-energy radiation, baking, and developing the exposed film in anorganic solvent-based developer to form a negative pattern wherein theunexposed region of resist film and the protective film are dissolvedaway and the exposed region of resist film is not dissolved.

In a second aspect, the invention provides a negative pattern-formingresist composition comprising a polymer and an organic solvent, thepolymer comprising recurring units (a1) of the above formula (1) andbeing dissolvable in a developer selected from among 2-octanone,2-nonanone, 2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone,3-hexanone, diisobutyl ketone, methylcyclohexanone, acetophenone,methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate,amyl acetate, isoamyl acetate, butenyl acetate, propyl formate, butylformate, isobutyl formate, amyl formate, isoamyl formate, methylvalerate, methyl pentenoate, methyl crotonate, ethyl crotonate, methylpropionate, ethyl propionate, ethyl 3-ethoxypropionate, methyl lactate,ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate, amyllactate, isoamyl lactate, methyl 2-hydroxyisobutyrate, ethyl2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate,benzyl acetate, methyl phenylacetate, benzyl formate, phenylethylformate, methyl 3-phenylpropionate, benzyl propionate, ethylphenylacetate, and 2-phenylethyl acetate.

The polymer may further comprise recurring units (a2) of the aboveformula (2) and recurring units of at least one type selected fromrecurring units (b1) to (b4) represented by the above formula (3). Thepolymer may further comprise recurring units derived from a monomerhaving an adhesive group selected from the group consisting of hydroxyl,cyano, carbonyl, ester, ether, lactone ring, carboxyl, carboxylicanhydride, sulfonic acid ester, disulfone, and carbonate. The polymermay further comprise recurring units (d1), (d2) or (d3) represented bythe above formula.

In a third aspect, the invention provides a polymer comprising recurringunits (a1) of the above formula (1) and having a weight averagemolecular weight of 1,000 to 100,000.

In a fourth aspect, the invention provides a monomer having the generalformula (1a):

wherein R¹ to R⁴ are as defined above.

Advantageous Effects of Invention

In a lithography process involving exposure, PEB and organic solventdevelopment, a resist composition comprising a polymer adapted to form alactone ring under the action of an acid displays a high dissolutioncontrast. A fine hole or trench pattern can be formed at a high degreeof size control and a high sensitivity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a patterning process according oneembodiment of the invention. FIG. 1A shows a photoresist film disposedon a substrate, FIG. 1B shows the resist film being exposed, and FIG. 1Cshows the resist film being developed in an organic solvent.

DESCRIPTION OF EMBODIMENTS

The terms “a” and “an” herein do not denote a limitation of quantity,but rather denote the presence of at least one of the referenced item.As used herein, the notation (C_(n)-C_(m)) means a group containing fromn to m carbon atoms per group. As used herein, the term “film” is usedinterchangeably with “coating” or “layer.”

The abbreviations and acronyms have the following meaning.

Mw: weight average molecular weight

Mn: number average molecular weight

Mw/Mn: molecular weight distribution or dispersity

GPC: gel permeation chromatography

PEB: post-exposure bake

PAG: photoacid generator

Me: methyl

Briefly stated, the invention pertains to a pattern forming processcomprising the steps of applying a resist composition comprising apolymer adapted to form a lactone ring under the action of an acid sothat the polymer may reduce its solubility in an organic solvent, anoptional acid generator and an organic solvent onto a substrate,prebaking to form a resist film, exposing to high-energy radiation forletting the acid generator (or acid-generating recurring unit) generatean acid and the polymer form a lactone ring, for thereby reducing thesolubility of the exposed region in an organic solvent-based developer,PEB, and developing in an organic solvent-based developer to form anegative pattern. The invention also pertains to a resist composition, apolymer, and a monomer.

It is generally known that lactone ring-containing polymers are lesssoluble in organic solvents. The polymer used in the pattern formingprocess of the invention is designed such that, under the action of thegenerated acid, it may form a lactone ring selectively in the exposedregion and thus reduce its solubility in organic solvents. This ensuresthat the film remains as a pattern. A change of lactone content beforeand after exposure brings about a substantial difference in dissolutionrate, achieving a high dissolution contrast upon organic solventdevelopment.

Great efforts are currently made to apply organic solvent development toArF resist compositions. In these ArF resist compositions, lactone unitsare incorporated into the base resin in as large a proportion as 30 to60 mol % in order to ensure the adhesion of pattern to substrate. Thisrestricts the solubility of the base resin itself in organic solventsand makes it essentially difficult to provide the unexposed region withthe desired high dissolution rate. By contrast, the polymer used in thepattern forming process of the invention is characterized in that alactone ring is newly formed in the exposed region where the film is tobe left as pattern and contributes to substrate adhesion. This enablesto reduce the amount of lactone initially introduced into the base resinand is effective in enhancing the dissolution rate of the base resin inthe unexposed region.

The recurring unit adapted to form a lactone ring under the action of anacid so that its solubility in an organic solvent may lower is arecurring unit (a1) having the general formula (1).

Herein R¹ is hydrogen or methyl. R² is hydrogen or a straight, branchedor cyclic monovalent hydrocarbon group of 1 to 20 carbon atoms in whicha constituent —CH₂— may be substituted by —O— or —C(═O)—. R³ is an acidlabile group. R⁴ is hydrogen or a straight, branched or cyclicmonovalent hydrocarbon group of 1 to 20 carbon atoms in which aconstituent —CH₂— may be substituted by —O— or —C(═O)—, and a1 is anumber in the range: 0<a1<1.0.

It is presumed that the recurring unit (a1) having formula (1) forms alactone ring under the action of an acid according to the followingschemes wherein R¹, R², R³, R⁴ and a1 are as defined above.

In formula (1), suitable monovalent hydrocarbon groups represented by R²and R⁴ include groups of the general formulae (R1-1) and (R1-2),tertiary alkyl groups of 4 to 15 carbon atoms, trialkylsilyl groups inwhich each alkyl moiety has 1 to 5 carbon atoms, oxoalkyl groups of 4 to15 carbon atoms, and acyl groups of 1 to 10 carbon atoms.

In these formulae, the broken line denotes a valence bond. In formula(R1-1), R^(L01) and R^(L02) each are hydrogen or a straight, branched orcyclic alkyl group of 1 to 18 carbon atoms, preferably 1 to 10 carbonatoms. Exemplary alkyl groups include methyl, ethyl, propyl, isopropyl,n-butyl, sec-butyl, tert-butyl, cyclopentyl, cyclohexyl, 2-ethylhexyl,n-octyl, norbornyl, tricyclodecanyl, tetracyclododecanyl, and adamantyl.R^(L03) is a monovalent hydrocarbon group of 1 to 18 carbon atoms,preferably 1 to 10 carbon atoms, which may contain a heteroatom such asoxygen, examples of which include straight, branched or cyclic alkylgroups and substituted forms of such alkyl groups in which some hydrogenatoms are replaced by hydroxyl, alkoxy, oxo, amino, alkylamino or thelike or which are separated by ether oxygen.

Suitable straight, branched or cyclic alkyl groups are as exemplifiedfor R^(L01) and R^(L02), and suitable substituted alkyl groups are shownbelow.

A pair of R^(L01) and R^(L02), R^(L01) and R^(L03), or R^(L02) andR^(L03) may bond together to form a ring with the carbon and oxygenatoms to which they are attached. Each of R^(L01), R^(L02) and R^(L03)is a straight or branched alkylene group of 1 to 18 carbon atoms,preferably 1 to 10 carbon atoms when they form a ring.

In formula (R1-2), R^(L04) is a tertiary alkyl group of 4 to 20 carbonatoms, preferably 4 to 15 carbon atoms, a trialkylsilyl group in whicheach alkyl moiety has 1 to 6 carbon atoms, an oxoalkyl group of 4 to 20carbon atoms, or a group of formula (R1-1).

Exemplary tertiary alkyl groups represented by R^(L04) are tert-butyl,tert-amyl, 1,1-diethylpropyl, 2-cyclopentylpropan-2-yl,2-cyclohexylpropan-2-yl, 2-(bicyclo[2.2.1]heptan-2-yl)propan-2-yl,2-(adamantan-1-yl)propan-2-yl, 1-ethylcyclopentyl, 1-butylcyclopentyl,1-ethylcyclohexyl, 1-butylcyclohexyl, 1-ethyl-2-cyclopentenyl,1-ethyl-2-cyclohexenyl, 2-methyl-2-adamantyl, and 2-ethyl-2-adamantyl.Exemplary trialkylsilyl groups are trimethylsilyl, triethylsilyl, anddimethyl-tert-butylsilyl. Exemplary oxoalkyl groups are 3-oxocyclohexyl,4-methyl-2-oxooxan-4-yl, and 5-methyl-2-oxooxolan-5-yl. Exemplary acylgroups include formyl, acetyl, ethylcarbonyl, pivaloyl, methoxycarbonyl,ethoxycarbonyl, tert-butoxycarbonyl, trifluoroacetyl, andtrichloroacetyl. Letter y is an integer of 0 to 6.

Of the protective groups having formula (R1-1), the straight or branchedgroups are exemplified by the following.

Of the protective groups of formula (R1-1), the cyclic ones are, forexample, tetrahydrofuran-2-yl, 2-methyltetrahydrofuran-2-yl,tetrahydropyran-2-yl, and 2-methyltetrahydropyran-2-yl.

Examples of the protective groups of formula (R1-2) includetert-butoxycarbonyl, tert-butoxycarbonylmethyl, tert-amyloxycarbonyl,tert-amyloxycarbonylmethyl, 1,1-diethylpropyloxycarbonyl,1,1-diethylpropyloxycarbonylmethyl, 1-ethyl cyclopentyloxycarbonyl,1-ethylcyclopentyloxycarbonylmethyl, 1-ethyl-2-cyclopentenyloxycarbonyl,1-ethyl-2-cyclopentenyloxycarbonylmethyl, 1-ethoxyethoxycarbonylmethyl,2-tetrahydropyranyloxycarbonylmethyl, and2-tetrahydrofuranyloxycarbonylmethyl.

The acid labile group represented by R³ in formula (1) will be describedlater.

Examples of the recurring unit having formula (1) are shown belowwherein R¹ is hydrogen or methyl.

In addition to the recurring units (a1) mentioned above, the polymerserving as base resin in the resist composition may have furthercopolymerized therein recurring units (a2) having the general formula(2).

Herein R¹⁰ is hydrogen or methyl. R¹¹ is an acid labile group differentfrom the acid labile group R³ in formula (1). X³ is a single bond,phenylene, naphthylene, or —C(═O)—O—R¹²—, wherein R¹² is a straight,branched or cyclic C₁-C₁₀ alkylene group which may have an ether moiety,ester moiety, lactone ring or hydroxyl moiety, or a phenylene ornaphthylene group, and a2 is a number in the range: 0≦a2<1.0.

When X³ stands for a C₁-C₁₀ alkylene group which may have an ether,ester, lactone ring or hydroxyl moiety, or a C₆-C₁₀ arylene group suchas phenylene or naphthylene, their examples are shown below.

In these formulae, the broken line denotes a valence bond.

The acid labile group represented by R¹¹ in formula (2) will bedescribed later.

In addition to the recurring units (a1) of formula (1) and the recurringunits (a2) of formula (2), the polymer may have further copolymerizedtherein recurring units having a hydroxyl group substituted with an acidlabile group. Suitable recurring units having a hydroxyl groupsubstituted with an acid labile group include units (b1) to (b4)represented by the general formula (3).

Herein R¹³ and R¹⁶ each are hydrogen or methyl. R¹⁴ is a straight,branched or cyclic, di- to pentavalent aliphatic hydrocarbon group of 1to 16 carbon atoms which may have an ether or ester moiety. R¹⁵ and R¹⁷each are an acid labile group. R¹⁸ to R²¹ and R²² to R²⁵ are eachindependently hydrogen, cyano, a straight, branched or cyclic C₁-C₆alkyl group, alkoxycarbonyl, or a group having an ether moiety orlactone ring, at least one of R¹⁸ to R²¹ and R²² to R²⁵ has a hydroxylgroup substituted with an acid labile group. The subscript m is aninteger of 1 to 4, n is 0 or 1, b1, b2, b3 and b4 are numbers in therange: 0≦b1<1.0, 0≦b2<1.0, 0≦b3<1.0, 0≦b4<1.0, and 0≦b1+b2+b3+b4<1.0.

Examples of the monomers from which recurring units (b1) and (b2) informula (3) are derived are given below. Herein R¹³ and R¹⁶ each arehydrogen or methyl, and R¹⁵ and R¹⁷ each are an acid labile group.

Examples of the monomers from which recurring units (b3) and (b4) arederived are given below. Herein R is an acid labile group.

The acid labile group represented by R³ in formula (1), the acid labilegroup represented by R¹¹ in formula (2) and the acid labile groups(substituting on hydroxyl) represented by R¹⁵, R¹⁷, any one of R¹⁸ toR²¹, and any one of R²² to R²⁵ in formula (3) may be the same ordifferent and selected from a variety of such groups, with the provisothat R³ and R¹¹ are different. Suitable acid labile groups includegroups of the following formula (AL-10), acetal groups of the followingformula (AL-11), tertiary alkyl groups of the following formula (AL-12),and oxoalkyl groups of 4 to 20 carbon atoms, but are not limitedthereto.

In formulae (AL-10) and (AL-11), R⁵¹ and R⁵⁴ each are a monovalenthydrocarbon group, typically straight, branched or cyclic alkyl group,of 1 to 40 carbon atoms, more specifically 1 to 20 carbon atoms, whichmay contain a heteroatom such as oxygen, sulfur, nitrogen or fluorine.R⁵² and R⁵³ each are hydrogen or a monovalent hydrocarbon group,typically straight, branched or cyclic C₁-C₂₀ alkyl group, which maycontain a heteroatom such as oxygen, sulfur, nitrogen or fluorine. Thesubscript “a5” is an integer of 0 to 10, and especially 1 to 5.Alternatively, a pair of R⁵² and R⁵³, R⁵² and R⁵⁴, or R⁵³ and R⁵⁴ maybond together to form a ring, specifically aliphatic ring, with thecarbon atom or the carbon and oxygen atoms to which they are attached,the ring having 3 to 20 carbon atoms, especially 4 to 16 carbon atoms.

In formula (AL-12), R⁵⁵, R⁵⁶ and R⁵⁷ each are a monovalent hydrocarbongroup, typically straight, branched or cyclic C₁-C₂₀ alkyl group, whichmay contain a heteroatom such as oxygen, sulfur, nitrogen or fluorine.Alternatively, a pair of R⁵⁵ and R⁵⁶, R⁵⁵ and R⁵⁷, or R⁵⁶ and R⁵⁷ maybond together to form a ring, specifically aliphatic ring, with thecarbon atom to which they are attached, the ring having 3 to 20 carbonatoms, especially 4 to 16 carbon atoms.

Illustrative examples of the groups of formula (AL-10) includetert-butoxycarbonyl, tert-butoxycarbonylmethyl, tert-amyloxycarbonyl,tert-amyloxycarbonylmethyl, 1-ethoxyethoxycarbonylmethyl,2-tetrahydropyranyloxycarbonylmethyl and2-tetrahydrofuranyloxycarbonylmethyl as well as substituent groups ofthe following formulae (AL-10)-1 to (AL-10)-10.

In formulae (AL-10)-1 to (AL-10)-10, R⁵⁵ is each independently astraight, branched or cyclic C₁-C₈ alkyl group, C₆-C₂₀ aryl group orC₇-C₂₀ aralkyl group; R⁵⁹ is hydrogen or a straight, branched or cyclicC₁-C₂₀ alkyl group; R⁶⁰ is a C₆-C₂₀ aryl group or C₇-C₂₀ aralkyl group;and “a5” is as defined above.

Illustrative examples of the acetal group of formula (AL-11) includethose of the following formulae (AL-11)-1 to (AL-11)-112.

Other examples of acid labile groups include those of the followingformula (AL-11a) or (AL-11b) while the polymer may be crosslinked withinthe molecule or between molecules with these acid labile groups.

Herein R⁶¹ and R⁶² each are hydrogen or a straight, branched or cyclicC₁-C₈ alkyl group, or R⁶¹ and R⁶² may bond together to form a ring withthe carbon atom to which they are attached, and R⁶¹ and R⁶² are straightor branched C₁-C₈ alkylene groups when they form a ring. R⁶³ is astraight, branched or cyclic C₁-C₁₀ alkylene group. Each of b5 and d5 is0 or an integer of 1 to 10, preferably 0 or an integer of 1 to 5, and c5is an integer of 1 to 7. “A” is a (c5+1)-valent aliphatic or alicyclicsaturated hydrocarbon group, aromatic hydrocarbon group or heterocyclicgroup having 1 to 50 carbon atoms, which may be separated by aheteroatom such as oxygen, sulfur or nitrogen or in which some hydrogenatoms attached to carbon atoms may be substituted by hydroxyl, carboxyl,carbonyl radicals or fluorine atoms. “B” is —CO—O—, —NHCO—O— or—NHCONH—.

Preferably, “A” is selected from divalent to tetravalent, straight,branched or cyclic C₁-C₂₀ alkylene, alkanetriyl and alkanetetraylgroups, and C₆-C₃₀ arylene groups, which may be separated by aheteroatom such as oxygen, sulfur or nitrogen or in which some hydrogenatoms attached to carbon atoms may be substituted by hydroxyl, carboxyl,acyl radicals or halogen atoms. The subscript c5 is preferably aninteger of 1 to 3.

The crosslinking acetal groups of formulae (AL-11a) and (AL-11b) areexemplified by the following formulae (AL-11)-113 through (AL-11)-120.

Illustrative examples of the tertiary alkyl of formula (AL-12) includetert-butyl, triethylcarbyl, 1-ethylnorbornyl, 1-methylcyclohexyl,1-ethylcyclopentyl, and tert-amyl groups as well as those of (AL-12)-1to (AL-12)-16.

Herein R⁶⁴ is each independently a straight, branched or cyclic C₁-C₈alkyl group, C₆-C₂₀ aryl group or C₇-C₂₀ aralkyl group, two R⁶⁴ may bondtogether to form a ring; R⁶⁵ and R⁶⁷ each are hydrogen, methyl or ethyl;and R⁶⁶ is a C₆-C₂₀ aryl group or C₇-C₂₀ aralkyl group.

With acid labile groups containing R⁶⁸ representative of a di- orpoly-valent alkylene or arylene group as shown by formula (AL-12)-17,the polymer may be crosslinked within the molecule or between molecules.In formula (AL-12)-17, R⁶⁴ is as defined above, R⁶⁸ is a single bond, ora straight, branched or cyclic C₁-C₂₀ alkylene group or arylene groupwhich may contain a heteroatom such as oxygen, sulfur or nitrogen, andb6 is an integer of 0 to 3. Formula (AL-12)-17 is applicable to all theforegoing acid labile groups.

The groups represented by R⁶⁴, R⁶⁵, R⁶⁶ and R⁶⁷ may contain a heteroatomsuch as oxygen, nitrogen or sulfur. Such groups are exemplified by thefollowing formulae (AL-13)-1 to (AL-13)-7.

While the polymer used herein preferably includes recurring units (a1)of formula (1), and optionally recurring units (a2) of formula (2) andacid labile group-containing recurring units (b1) to (b4) of formula(3), it may have further copolymerized therein recurring units (c)derived from a monomer having an adhesive group such as hydroxyl, cyano,carbonyl, ester, ether, lactone ring, carboxyl, carboxylic anhydride,sulfonic acid ester, disulfone, and carbonate group. Inter alia,monomers having a lactone ring as the adhesive group are most preferred.

Examples of monomers from which recurring units (c) are derived aregiven below.

In a preferred embodiment, the polymer has further copolymerized thereinrecurring units of at least one type selected from sulfonium salt units(d1) to (d3) represented by the general formulae below.

Herein R⁰²⁰, R⁰²⁴, and R⁰²⁸ each are hydrogen or methyl; R⁰²¹ is asingle bond, phenylene, —O—R⁰³³—, or —C(═O)—Y—R⁰³³—, wherein Y is oxygenor NH and R⁰³³ is a straight, branched or cyclic C₁-C₆ alkylene group,alkenylene group or phenylene group, which may contain a carbonyl(—CO—), ester (—COO—), ether (—O—), or hydroxyl moiety; R⁰²², R⁰²³,R⁰²⁵, R⁰²⁶, R⁰²⁷, R⁰²⁹, R⁰³⁰, and R⁰³¹ are each independently astraight, branched or cyclic C₁-C₁₂ alkyl group which may contain acarbonyl, ester or ether moiety, a C₆-C₁₂ aryl group, a C₇-C₂₀ aralkylgroup, or a thiophenyl group; Z¹ is a single bond, methylene, ethylene,phenylene, fluorinated phenylene, —O—R⁰³²—, or —C(═O)—Z²—R⁰³²—, whereinZ² is oxygen or NH, and R⁰³² is a straight, branched or cyclic C₁-C₆alkylene group, alkenylene group or phenylene group, which may contain acarbonyl, ester, ether or hydroxyl moiety; M⁻ is a non-nucleophiliccounter ion; d1 to d4 are in the range: 0≦d1≦0.3, 0≦d2≦0.3, 0≦d3≦0.3,and 0≦d1+d2+d3≦0.3.

Besides the recurring units described above, the polymer may havefurther copolymerized therein additional recurring units, for example,recurring units (e) having a non-leaving hydrocarbon group as describedin JP-A 2008-281980. Examples of the non-leaving hydrocarbon group otherthan those described in JP-A 2008-281980 include indene, acenaphthylene,and norbornadiene derivatives. Copolymerization of recurring units (e)having a non-leaving hydrocarbon group is effective for improving thedissolution of the polymer in organic solvent-based developer.

It is also possible to incorporate recurring units (f) having an oxiraneor oxetane ring into the polymer. Where recurring units (f) having anoxirane or oxetane ring are copolymerized in the polymer, the exposedregion of resist film will be crosslinked, leading to improvements infilm retention of the exposed region and etch resistance. Examples ofthe recurring units (f) having an oxirane or oxetane ring are givenbelow wherein R⁴¹ is hydrogen or methyl.

Appropriate molar fractions of individual recurring units (a1), (a2),(b1), (b2), (b3), (b4), (c), (d1), (d2), (d3), (e), and (f) incorporatedin the polymer are in the range: 0<a1<1.0, 0≦a2<1.0, 0≦b1<1.0, 0≦b2<1.0,0≦b3<1.0, 0≦b4<1.0, 0≦a2+b1+b2+b3+b4<1.0, 0<c<1.0, 0≦d1≦0.3, 0≦d2≦0.3,0≦d3≦0.3, 0≦d1+d2+d3≦0.3, 0≦e 0.4, 0≦f≦0.6; preferably 0.1≦a1≦0.9,0≦a2≦0.9, 0.1≦a1+a2≦0.9, 0≦b1≦0.9, 0≦b2≦0.9, 0≦b3≦0.9, 0≦b4≦0.9,0.1≦a1+a2+b1+b2+b3+b4≦0.9, 0.1≦c 0.9, 0≦d1≦0.2, 0≦d2≦0.2, 0≦d3≦0.2,0≦d1+d2+d3≦0.2, 0≦e 0.3, 0≦f≦0.5, provided thata1+a2+b1+b2+b3+b4+c+d1+d2+d3+e+f=1.

The polymer serving as the base resin in the resist composition used inthe pattern forming process of the invention should preferably have aweight average molecular weight (Mw) in the range of 1,000 to 500,000,and more preferably 2,000 to 30,000, as measured by GPC versuspolystyrene standards using tetrahydrofuran solvent. With too low a Mw,a film thickness loss is likely to occur upon organic solventdevelopment. A polymer with too high a Mw may lose solubility in organicsolvent and have a likelihood of footing after pattern formation.

If a polymer has a wide molecular weight distribution or dispersity(Mw/Mn), which indicates the presence of lower and higher molecularweight polymer fractions, there is a possibility that followingexposure, foreign matter is left on the pattern or the pattern profileis exacerbated. The influences of molecular weight and dispersity becomestronger as the pattern rule becomes finer. Therefore, themulti-component copolymer should preferably have a narrow dispersity(Mw/Mn) of 1.0 to 2.0, especially 1.0 to 1.5, in order to provide aresist composition suitable for micropatterning to a small feature size.

The polymer used herein may be synthesized by any desired method, forexample, by dissolving unsaturated bond-containing monomerscorresponding to the respective recurring units in an organic solvent,adding a radical initiator thereto, and effecting heat polymerization.Examples of the organic solvent which can be used for polymerizationinclude toluene, benzene, tetrahydrofuran, diethyl ether and dioxane.Examples of the polymerization initiator used herein include2,2′-azobisisobutyronitrile (AIBN),2,2′-azobis(2,4-dimethylvaleronitrile), dimethyl2,2-azobis(2-methylpropionate), benzoyl peroxide, and lauroyl peroxide.Preferably the system is heated at 50 to 80° C. for polymerization totake place. The reaction time is 2 to 100 hours, preferably 5 to 20hours. The acid labile group that has been incorporated in the monomermay be kept as such, or the product may be protected or partiallyprotected after polymerization.

For example, a polymer comprising recurring units of formula (1) may besynthesized by using a polymerizable double bond-bearing ester compoundof the general formula (1a) as the monomer, and effecting polymerizationthereof.

Herein R¹ is hydrogen or methyl. R² is hydrogen or a straight, branchedor cyclic monovalent hydrocarbon group of 1 to 20 carbon atoms in whicha constituent —CH₂— may be substituted by —O— or —C(═O)—. R³ is an acidlabile group. R⁴ is hydrogen or a straight, branched or cyclicmonovalent hydrocarbon group of 1 to 20 carbon atoms in which aconstituent —CH₂— may be substituted by —O— or —C(═O)—.

Examples of the polymerizable monomer having formula (1a) are givenbelow, but not limited thereto. Herein R¹ is hydrogen or methyl.

The synthesis of the monomer having formula (1a) is not particularlylimited. It may be synthesized by the optimum method selected dependingon its structure. For example, the monomer having formula (1a) whereinR² is hydrogen may be synthesized as shown by the following scheme.

Herein R¹ to R⁴ are as defined above, X⁴ is hydrogen or halogen, M isLi, Na, K, MgY or ZnY, and Y is halogen.

First, a base acts on a corresponding acetate (where X⁴ is hydrogen) orhaloacetate (where X⁴ is halogen) to form a metal enolate (5). Thennucleophilic addition reaction is effected between the metal enolate (5)and a carbonyl compound (6) to form the desired monomer (1a).

The bases used herein include metal amides such as sodium amide,potassium amide, lithium diisopropylamide, potassium diisopropylamide,lithium dicyclohexylamide, potassium dicyclohexylamide, lithium2,2,6,6-tetramethylpiperidine, lithium bistrimethylsilylamide, sodiumbistrimethylsilylamide, potassium bistrimethylsilylamide, lithiumisopropylcyclohexylamide, and bromomagnesium diisopropylamide; alkoxidessuch as sodium methoxide, sodium ethoxide, lithium methoxide, lithiumethoxide, lithium tert-butoxide, and potassium tert-butoxide; inorganichydroxides such as sodium hydroxide, lithium hydroxide, potassiumhydroxide, barium hydroxide, and tetra-n-butylammonium hydroxide;inorganic carbonates such as sodium carbonate, sodium hydrogencarbonate,lithium carbonate and potassium carbonate; metal hydrides such asboranes, alkylboranes, sodium hydride, lithium hydride, potassiumhydride, and calcium hydride; alkyl metal compounds such as trityllithium, trityl sodium, trityl potassium, methyl lithium, phenyllithium, sec-butyl lithium, tert-butyl lithium, and ethyl magnesiumbromide; and metals such as lithium, sodium, potassium, magnesium, andzinc, but are not limited thereto. It is noted that reaction usinghaloacetate and zinc is known as Reformatsky reaction.

The intermediate, carbonyl compound (6) may be prepared, for example,via steps i) to iii) according to the reaction scheme shown belowalthough the synthesis route is not limited thereto.

Herein R¹ and R³ are as defined above, X⁵ is halogen, hydroxyl or —OR⁵,R⁵ is methyl, ethyl or a group of the formula (12):

X⁶ is halogen, and Me is methyl.

Step i) is a reaction of diol compound (7) with esterifying agent (8) toform hydroxy-ester compound (9). One reactant, diol compound (7) may bereadily prepared by the method described in Journal of OrganicChemistry, Vol. 64, p. 4943 (1999), as Non-Patent Document 1.

The reaction may readily run by a well-known procedure. The preferredesterifying agent (8) is an acid chloride of formula (8) wherein X⁵ ischlorine or a carboxylic anhydride of formula (8) wherein X⁵ is —OR⁵ andR⁵ is a group of formula (12). When an acid chloride, typicallycarboxylic acid chloride such as methacrylic acid chloride is used asthe esterifying agent (8), the reaction may be conducted in asolventless system or in a solvent (e.g., methylene chloride,acetonitrile, toluene or hexane) by adding diol compound (7), acidchloride, and a base (e.g., triethylamine, pyridine or4-dimethylaminopyridine) in sequence or at a time, and optional coolingor heating. When a carboxylic anhydride such as methacrylic anhydride isused as the esterifying agent (8), the reaction may be conducted in asolventless system or in a solvent (e.g., methylene chloride,acetonitrile, toluene or hexane) by adding diol compound (7), carboxylicanhydride, and a base (e.g., triethylamine, pyridine or4-dimethylaminopyridine) in sequence or at a time, and optional coolingor heating.

An appropriate amount of diol compound (7) used is 1 to 10 moles, morepreferably 1 to 5 moles per mole of esterifying agent (8). Less than 1mole of diol compound (7) may result in a substantial drop of percentyield because a noticeable amount of a bis-ester compound may be formedby side reaction to esterify both the hydroxyl groups of diol compound(7). More than 10 moles of diol compound (7) may be uneconomical becauseof an increase of reactant cost and a lowering of pot yield.

The reaction time is determined as appropriate by monitoring thereaction process by thin-layer chromatography (TLC) or gaschromatography (GC) because it is desirable from the yield aspect todrive the reaction to completion. Usually the reaction time is about 30minutes to about 40 hours. Hydroxy-ester compound (9) may be recoveredfrom the reaction mixture by ordinary aqueous work-up. If necessary, itmay be purified by standard techniques like distillation,recrystallization and chromatography.

Step ii) is hydrolysis of dimethylacetal group of hydroxy-ester compound(9) under acidic conditions to form hydroxy-keto-ester compound (10).The reaction may readily run by a well-known procedure. Preferably anacid catalyst is used. Suitable acid catalysts include mineral acidssuch as hydrochloric acid, sulfuric acid, nitric acid and perchloricacid and organic acids such as trifluoroacetic acid, benzenesulfonicacid, p-toluenesulfonic acid and trifluoromethanesulfonic acid.

Step iii) is a reaction of hydroxy-keto-ester compound (10) withprotecting agent (11) to form carbonyl compound (9). The protectingagent (11) has the formula: R³—X⁶ wherein R³ is as defined above and X⁶is a halogen. Exemplary halogen atoms are chlorine, bromine and iodine,with chlorine being most preferred for ease of handling.

The reaction may readily run by a well-known procedure. Where R³ informula (6) is methoxymethyl, preferably the reaction may be conductedin a solventless system or in a solvent by adding hydroxy-keto-estercompound (10), protecting agent (11) (e.g., chloromethyl methyl ether),and a base (e.g., triethylamine, pyridine, N,N-diisopropylethylamine or4-dimethylaminopyridine) in sequence or at a time, and optional coolingor heating.

An appropriate amount of protecting agent (11) used is 0.5 to 10 moles,more preferably 1.0 to 3.0 moles per mole of hydroxy-keto-ester compound(10). Less than 0.5 mole of protecting agent (11) may result in asubstantial drop of percent yield because a large fraction of thereactant may be left unreacted. More than 10 moles of protecting agent(11) may be uneconomical because of an increase of reactant cost and alowering of pot yield.

Suitable solvents used in step iii) include hydrocarbons such astoluene, xylene, hexane, and heptane; chlorinated solvents such asmethylene chloride, chloroform and dichloroethane; ethers such asdiethyl ether, tetrahydrofuran and dibutyl ether; ketones such asacetone and 2-butanone; esters such as ethyl acetate and butyl acetate;nitriles such as acetonitrile; alcohols such as methanol and ethanol;aprotic polar solvents such as N,N-dimethylformamide,N,N-dimethylacetamide and dimethyl sulfoxide; and water, which may beused alone or in admixture. To the reaction system, a phase transfercatalyst such as tetrabutylammonium hydrogensulfate may be added. Anappropriate amount of the phase transfer catalyst, if used, is 0.0001 to1.0 mole, more preferably 0.001 to 0.5 mole per mole ofhydroxy-keto-ester compound (10). Less than 0.0001 mole of the catalystmay fail to achieve an addition effect whereas more than 1.0 mole of thecatalyst may be uneconomical because of an increased catalyst cost.

The reaction time is determined as appropriate by monitoring thereaction process by TLC or GC because it is desirable from the yieldaspect to drive the reaction to completion. Usually the reaction time isabout 30 minutes to about 40 hours. The carbonyl compound (6) may berecovered from the reaction mixture by ordinary aqueous work-up. Ifnecessary, the compound may be purified by standard techniques likedistillation, recrystallization and chromatography.

When it is desired to obtain carbonyl compound (6) wherein R³ is atertiary alkyl group such as tert-butyl, tert-amyl, methylcyclopentyl,ethylcyclopentyl, methylcyclohexyl, ethylcyclohexyl, methyladamantyl orethyladamantyl, step iii) may follow an alternative route. That is,reaction of hydroxy-keto-ester compound (10) with a corresponding olefin(e.g., isobutene or isoamylene) may be conducted in a solventless systemor in a solvent (e.g., toluene or hexane) in the presence of an acidcatalyst at a temperature of −20° C. to 50° C. Suitable acid catalystsinclude mineral acids such as hydrochloric acid, sulfuric acid, nitricacid and perchloric acid, and organic acids such as methanesulfonicacid, trifluoromethanesulfonic acid, p-toluenesulfonic acid andbenzenesulfonic acid.

It is acceptable to use a blend of two or more inventive polymers whichdiffer in compositional ratio, molecular weight or dispersity as thebase resin in the resist composition. Also useful is a blend of aninventive polymer comprising recurring units of formula (1) and apolymer comprising recurring units having a conventional acid labilegroup-substituted hydroxyl or carboxyl group, for example, a polymercomprising recurring units (a2) and/or (b1) to (b4).

In some embodiments, the inventive polymer may be blended with a polymerof the conventional type wherein the exposed region of resist film isdissolved on alkaline development such as (meth)acrylate polymer,polynorbornene, cycloolefin-maleic anhydride copolymer, or ring-openingmetathesis polymerization (ROMP) polymer. Also, the inventive polymermay be blended with a (meth)acrylate polymer, polynorbornene orcycloolefin-maleic anhydride copolymer having an acid labilegroup-substituted hydroxyl group wherein the exposed region of resistfilm is not dissolved by alkaline development, but negative pattern isformed by organic solvent development.

Resist Composition

The resist composition used in the pattern forming process of theinvention comprises the polymer defined above as base resin, an organicsolvent, and optionally, a compound capable of generating an acid inresponse to high-energy radiation (known as “acid generator”), basiccompound, dissolution regulator, surfactant, acetylene alcohol, andother components.

The resist composition used herein may include an acid generator inorder for the composition to function as a chemically amplified positiveresist composition. Typical of the acid generator used herein is aphotoacid generator (PAG) capable of generating an acid in response toactinic light or radiation. The PAG may preferably be compounded in anamount of 0.5 to 30 parts and more preferably 1 to 20 parts by weightper 100 parts by weight of the base resin. The PAG is any compoundcapable of generating an acid upon exposure to high-energy radiation.Suitable PAGs include sulfonium salts, iodonium salts,sulfonyldiazomethane, N-sulfonyloxyimide, and oxime-O-sulfonate acidgenerators. The PAGs may be used alone or in admixture of two or more.Typically acid generators generate such acids as sulfonic acids, imidicacids and methide acids. Of these, sulfonic acids which are fluorinatedat α-position are most commonly used. In case the acid labile group isan acetal group which is susceptible to deprotection, the sulfonic acidneed not necessarily be fluorinated at α-position. In the embodimentwherein the polymer has recurring units (d1), (d2) or (d3) of acidgenerator copolymerized therein, the acid generator need not beseparately added.

Examples of the organic solvent used herein are described in JP-A2008-111103, paragraphs [0144] to [0145] (U.S. Pat. No. 7,537,880).Specifically, exemplary solvents include ketones such as cyclohexanoneand methyl-2-n-amyl ketone; alcohols such as 3-methoxybutanol,3-methyl-3-methoxybutanol, 1-methoxy-2-propanol, and1-ethoxy-2-propanol; ethers such as propylene glycol monomethyl ether,ethylene glycol monomethyl ether, propylene glycol monoethyl ether,ethylene glycol monoethyl ether, propylene glycol dimethyl ether, anddiethylene glycol dimethyl ether; esters such as propylene glycolmonomethyl ether acetate (PGMEA), propylene glycol monoethyl etheracetate, ethyl lactate, ethyl pyruvate, butyl acetate, methyl3-methoxypropionate, ethyl 3-ethoxypropionate, tert-butyl acetate,tert-butyl propionate, and propylene glycol mono-tert-butyl etheracetate; and lactones such as γ-butyrolactone, and mixtures thereof.Where an acid labile group of acetal form is used, a high-boilingalcohol solvent such as diethylene glycol, propylene glycol, glycerol,1,4-butane diol or 1,3-butane diol may be added for acceleratingdeprotection reaction of acetal.

Basic compounds, typically amines may be added to the resistcomposition. Suitable basic compounds include primary, secondary andtertiary amine compounds, specifically amine compounds having ahydroxyl, ether, ester, lactone, cyano or sulfonate group, as describedin JP-A 2008-111103, paragraphs to [0164], and compounds having acarbamate group, as described in JP 3790649.

Onium salts such as sulfonium salts, iodonium salts and ammonium saltsof sulfonic acids which are not fluorinated at α-position as describedin US 20080153030 (JP-A 2008-158339) and similar onium salts ofcarboxylic acid as described in JP 3991462 may be used as the quencher.Where the acid labile group is an acetal group which is very sensitiveto acid, the acid for eliminating the protective group need notnecessarily be an α-position fluorinated sulfonic acid, imidic acid, ormethidic acid because sometimes deprotection reaction may proceed evenwith an α-position non-fluorinated sulfonic acid. In this case, an oniumsalt of sulfonic acid cannot be used as the quencher, and instead, anonium salt of carboxylic acid is preferably used alone.

Exemplary surfactants are described in JP-A 2008-111103, paragraphs[0165] to [0166]. Exemplary dissolution regulators are described in JP-A2008-122932 (US 2008090172), paragraphs [0155] to [0178], and exemplaryacetylene alcohols in paragraphs [0179] to [0182].

Also a polymeric additive may be added for improving the waterrepellency on surface of a resist film as spin coated. This additive maybe used in the topcoatless immersion lithography. These additives have aspecific structure with a 1,1,1,3,3,3-hexafluoro-2-propanol residue andare described in JP-A 2007-297590, JP-A 2008-111103 and JP-A2012-128067. The water repellency improver to be added to the resistshould be soluble in the organic solvent as developer. The waterrepellency improver of specific structure with a1,1,1,3,3,3-hexafluoro-2-propanol residue is well soluble in thedeveloper. A polymer having an amino group or amine salt copolymerizedas recurring units may serve as the water repellent additive and iseffective for preventing evaporation of acid during PEB and avoiding anyhole pattern opening failure after development. An appropriate amount ofthe water repellency improver is 0.1 to 20 parts, preferably 0.5 to 10parts by weight per 100 parts by weight of the base resin.

Notably, an appropriate amount of the organic solvent is 100 to 10,000parts, preferably 300 to 8,000 parts by weight, and an appropriateamount of the basic compound is 0.0001 to 30 parts, preferably 0.001 to20 parts by weight, per 100 parts by weight of the base resin. Amountsof the dissolution regulator, surfactant, and acetylene alcohol may bedetermined as appropriate depending on their purpose of addition.

Process

One embodiment of the invention is a pattern forming process comprisingthe steps of coating a resist composition onto a substrate, prebakingthe resist composition to form a resist film, exposing a selected regionof the resist film to high-energy radiation, baking (PEB), anddeveloping the exposed resist film with an organic solvent developer sothat the unexposed region of resist film is dissolved and the exposedregion of resist film is left, thereby forming a negative tone resistpattern such as a hole or trench pattern.

FIG. 1 illustrates the pattern forming process of the invention. First,the positive resist composition is coated on a substrate to form aresist film thereon. Specifically, a resist film 40 of a positive resistcomposition is formed on a processable substrate 20 disposed on asubstrate 10 directly or via an intermediate intervening layer 30 asshown in FIG. 1A. The resist film preferably has a thickness of 10 to1,000 nm and more preferably 20 to 500 nm. Prior to exposure, the resistfilm is heated or prebaked, preferably at a temperature of 60 to 180°C., especially 70 to 150° C. for a time of 10 to 300 seconds, especially15 to 200 seconds.

The substrate 10 used herein is generally a silicon substrate. Theprocessable substrate (or target film) 20 used herein includes SiO₂,SiN, SiON, SiOC, p-Si, α-Si, TiN, WSi, BPSG, SOG, Cr, CrO, CrON, MoSi,low dielectric film, and etch stopper film. The intermediate interveninglayer 30 includes hard masks of SiO₂, SiN, SiON or p-Si, an undercoat inthe form of carbon film, a silicon-containing intermediate film, and anorganic antireflective coating.

Next comes exposure depicted at 50 in FIG. 1B. For the exposure,preference is given to high-energy radiation having a wavelength of 140to 250 nm, EUV having a wavelength of 13.5 nm, and EB, and especiallyArF excimer laser radiation of 193 nm. The exposure may be done eitherin a dry atmosphere such as air or nitrogen stream or by immersionlithography in water. The ArF immersion lithography uses deionized wateror liquids having a refractive index of at least 1 and highlytransparent to the exposure wavelength such as alkanes as the immersionsolvent. The immersion lithography involves prebaking a resist film andexposing the resist film to light through a projection lens, with wateror liquid introduced between the resist film and the projection lens.Since this allows lenses to be designed to a NA of 1.0 or higher,formation of finer feature size patterns is possible. The immersionlithography is important for the ArF lithography to survive to the 45-nmnode. In the case of immersion lithography, deionized water rinsing (orpost-soaking) may be carried out after exposure for removing waterdroplets left on the resist film, or a protective film may be appliedonto the resist film after pre-baking for preventing any leach-out fromthe resist film and improving water slip on the film surface.

The resist protective film used in the immersion lithography ispreferably formed from a solution of a polymer having1,1,1,3,3,3-hexafluoro-2-propanol residues which is insoluble in water,but soluble in an alkaline developer liquid, in a solvent selected fromalcohols of at least 4 carbon atoms, ethers of 8 to 12 carbon atoms, andmixtures thereof. The protective film-forming composition used hereinmay be based on a polymer comprising recurring units derived from amonomer having a 1,1,1,3,3,3-hexafluoro-2-propanol residue. While theprotective film must dissolve in the organic solvent developer, thepolymer comprising recurring units derived from a monomer having a1,1,1,3,3,3-hexafluoro-2-propanol residue dissolves in organic solventdevelopers. In particular, protective film-forming materials having1,1,1,3,3,3-hexafluoro-2-propanol residues as described in JP-A2007-025634 and JP-A 2008-003569 readily dissolve in organic solventdevelopers.

In the protective film-forming composition, an amine compound or aminesalt or a polymer having copolymerized therein recurring unitscontaining an amine compound or amine salt may be used. This componentis effective for controlling diffusion of the acid generated in theexposed region of the photoresist film to the unexposed region forthereby preventing any hole opening failure. Useful protective filmmaterials having an amine compound added thereto are described in JP-A2008-003569, and useful protective film materials having an amino groupor amine salt copolymerized are described in JP-A 2007-316448. The aminecompound or amine salt may be selected from the compounds enumerated asthe basic compound to be added to the resist composition. An appropriateamount of the amine compound or amine salt added is 0.01 to 10 parts,preferably 0.02 to 8 parts by weight per 100 parts by weight of the baseresin.

After formation of the photoresist film, deionized water rinsing (orpost-soaking) may be carried out for extracting the acid generator andthe like from the film surface or washing away particles, or afterexposure, rinsing (or post-soaking) may be carried out for removingwater droplets left on the resist film. If the acid evaporating from theexposed region during PEB deposits on the unexposed region to deprotectthe protective group on the surface of the unexposed region, there is apossibility that the surface edges of holes after development arebridged to close the holes. Particularly in the case of negativedevelopment, regions surrounding the holes receive light so that acid isgenerated therein. There is a possibility that the holes are not openedif the acid outside the holes evaporates and deposits inside the holesduring PEB. Provision of a protective film is effective for preventingevaporation of acid and for avoiding any hole opening failure. Aprotective film having an amine compound added thereto is more effectivefor preventing acid evaporation. On the other hand, a protective film towhich an acid compound such as a carboxyl or sulfo group is added orwhich is based on a polymer having copolymerized therein monomeric unitscontaining a carboxyl or sulfo group is undesirable because of apotential hole opening failure.

The other embodiment of the invention is a process for forming a patternby applying a resist composition comprising a polymer comprising acidlabile group-substituted recurring units of formula (1), an optionalacid generator, and an organic solvent onto a substrate, baking thecomposition to form a resist film, forming a protective film on theresist film, exposing the resist film to high-energy radiation to defineexposed and unexposed regions, PEB, and applying an organic solvent tothe coated substrate to form a negative pattern wherein the unexposedregion of resist film and the protective film are dissolved and theexposed region of resist film is not dissolved. The protective film ispreferably formed from a composition comprising a polymer bearing a1,1,1,3,3,3-hexafluoro-2-propanol residue and an amino group or aminesalt-containing compound, or a composition comprising a polymer bearinga 1,1,1,3,3,3-hexafluoro-2-propanol residue and having amino group oramine salt-containing recurring units copolymerized, the compositionfurther comprising an alcohol solvent of at least 4 carbon atoms, anether solvent of 8 to 12 carbon atoms, or a mixture thereof.

Examples of suitable recurring units having a1,1,1,3,3,3-hexafluoro-2-propanol residue include those derived fromhydroxyl-bearing monomers selected from among the monomers listed forunits (c) on pages 53, 54 and 55. Examples of the amino group-containingcompound include the amine compounds described in JP-A 2008-111103,paragraphs [0146] to [0164] as being added to photoresist compositions.Examples of the amine salt-containing compound include salts of theforegoing amine compounds with carboxylic acids or sulfonic acids.

Suitable alcohols of at least 4 carbon atoms include 1-butyl alcohol,2-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, 1-pentanol,2-pentanol, 3-pentanol, tert-amyl alcohol, neopentyl alcohol,2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-3-pentanol,cyclopentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2,3-dimethyl-2-butanol,3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol,2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol,3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol,4-methyl-1-pentanol, 4-methyl-2-pentanol, 4-methyl-3-pentanol,cyclohexanol, and 1-octanol. Suitable ether solvents of 8 to 12 carbonatoms include di-n-butyl ether, diisobutyl ether, di-sec-butyl ether,di-n-pentyl ether, diisopentyl ether, di-sec-pentyl ether, di-tert-amylether, and di-n-hexyl ether.

Exposure is preferably performed in an exposure dose of about 1 to 200mJ/cm², more preferably about 10 to 100 mJ/cm². This is followed bybaking (PEB) on a hot plate at 60 to 150° C. for 1 to 5 minutes,preferably at 80 to 120° C. for 1 to 3 minutes.

Thereafter the exposed resist film is developed in a developerconsisting of an organic solvent for 0.1 to 3 minutes, preferably 0.5 to2 minutes by any conventional techniques such as dip, puddle and spraytechniques. In this way, the unexposed region of resist film wasdissolved away, leaving a negative resist pattern 40 on the substrate 10as shown in FIG. 1C. The developer used herein is preferably selectedfrom among ketones such as 2-octanone, 2-nonanone, 2-heptanone,3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone,methylcyclohexanone, acetophenone, and methylacetophenone, and esterssuch as propyl acetate, butyl acetate, isobutyl acetate, amyl acetate,butenyl acetate, isoamyl acetate, propyl formate, butyl formate,isobutyl formate, amyl formate, isoamyl formate, methyl valerate, methylpentenoate, methyl crotonate, ethyl crotonate, methyl propionate, ethylpropionate, ethyl 3-ethoxypropionate, methyl lactate, ethyl lactate,propyl lactate, butyl lactate, isobutyl lactate, amyl lactate, isoamyllactate, methyl 2-hydroxyisobutyrate, ethyl 2-hydroxyisobutyrate, methylbenzoate, ethyl benzoate, phenyl acetate, benzyl acetate, methylphenylacetate, benzyl formate, phenylethyl formate, methyl3-phenylpropionate, benzyl propionate, ethyl phenylacetate, and2-phenylethyl acetate, and mixtures thereof. One or more of thesesolvents may be used as the developer. When a mixture of plural solventsis used, they may be mixed in any desired ratio. A surfactant may beadded to the developer while it may be selected from the same list ofcompounds as exemplified for the surfactant to be added to the resistcomposition.

At the end of development, the resist film is rinsed. As the rinsingliquid, a solvent which is miscible with the developer and does notdissolve the resist film is preferred. Suitable solvents includealcohols of 3 to 10 carbon atoms, ether compounds of 8 to 12 carbonatoms, alkanes, alkenes, and alkynes of 6 to 12 carbon atoms, andaromatic solvents. Specifically, suitable alkanes of 6 to 12 carbonatoms include hexane, heptane, octane, nonane, decane, undecane,dodecane, methylcyclopentane, dimethylcyclopentane, cyclohexane,methylcyclohexane, dimethylcyclohexane, cycloheptane, cyclooctane, andcyclononane. Suitable alkenes of 6 to 12 carbon atoms include hexene,heptene, octene, cyclohexene, methylcyclohexene, dimethylcyclohexene,cycloheptene, and cyclooctene. Suitable alkynes of 6 to 12 carbon atomsinclude hexyne, heptyne, and octyne. Suitable alcohols of 3 to 10 carbonatoms include n-propyl alcohol, isopropyl alcohol, 1-butyl alcohol,2-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, 1-pentanol,2-pentanol, 3-pentanol, tert-amyl alcohol, neopentyl alcohol,2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-3-pentanol,cyclopentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2,3-dimethyl-2-butanol,3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol,2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol,3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol,4-methyl-1-pentanol, 4-methyl-2-pentanol, 4-methyl-3-pentanol,cyclohexanol, and 1-octanol. Suitable ether compounds of 8 to 12 carbonatoms include di-n-butyl ether, diisobutyl ether, di-sec-butyl ether,di-n-pentyl ether, diisopentyl ether, di-sec-pentyl ether, di-tert-amylether, and di-n-hexyl ether. The solvents may be used alone or inadmixture. Besides the foregoing solvents, aromatic solvents may beused, for example, toluene, xylene, ethylbenzene, isopropylbenzene,tert-butylbenzene and mesitylene. Rinsing is effective for minimizingthe risks of resist pattern collapse and defect formation. However,rinsing is not essential. If rinsing is omitted, the amount of solventused may be reduced.

A hole pattern after reversal may be shrunk by the RELACS® process. Ahole pattern is shrunk by coating a shrink agent thereto, and bakingsuch that the shrink agent may undergo crosslinking at the resistsurface as a result of the acid catalyst diffusing from the resist layerduring bake, and the shrink agent may attach to the sidewall of the holepattern. The bake is at a temperature of 70 to 180° C., preferably 80 to170° C., for a time of 10 to 300 seconds. The extra shrink agent isstripped and the hole pattern is shrunk.

Where a hole pattern is formed by negative tone development, exposure bydouble dipole illuminations of X- and Y-direction line patterns providesthe highest contrast light. The contrast may be further increased bycombining dipole illumination with s-polarized illumination.

When a halftone phase shift mask bearing a lattice-like shifter patternis used, a pattern of holes may be formed at the intersections betweengratings of the lattice-like shifter pattern after development, asdescribed in JP-A 2011-170316, paragraph [0097] (US 20110177462). Thepreferred halftone phase shift mask bearing a lattice-like shifterpattern has a transmittance of 3 to 15%. More preferably, the phaseshift mask used is a phase shift mask including a lattice-like firstshifter having a line width equal to or less than a half pitch and asecond shifter arrayed on the first shifter and consisting of lineswhose on-wafer size is 2 to 30 nm thicker than the line width of thefirst shifter, whereby a pattern of holes is formed only where the thickshifter is arrayed. Also preferably, the phase shift mask used is aphase shift mask including a lattice-like first shifter having a linewidth equal to or less than a half pitch and a second shifter arrayed onthe first shifter and consisting of dots whose on-wafer size is 2 to 100nm thicker than the line width of the first shifter, whereby a patternof holes is formed only where the thick shifter is arrayed.

Exposure by double dipole illuminations of X- and Y-direction linescombined with polarized illumination presents a method of forming lightof the highest contrast. This method, however, has the drawback that thethroughput is substantially reduced by double exposures and maskexchange therebetween. To continuously carry out two exposures whileexchanging a mask, the exposure tool must be equipped with two maskstages although the existing exposure tool includes a single mask stage.Higher throughputs may be obtained by carrying out exposure of Xdirection lines continuously on 25 wafers in a front-opening unified pod(FOUP), exchanging the mask, and carrying out exposure continuously onthe same 25 wafers, rather than exchanging a mask on every exposure of asingle wafer. However, a problem arises that as the time duration untilthe first one of 25 wafers is exposed in the second exposure isprolonged, the environment affects the resist such that the resist afterdevelopment may change its size and shape. To block the environmentalimpact on wafers in standby until the second exposure, it is effectivethat the resist film is overlaid with a protective film.

To proceed with a single mask, it is proposed in Non-Patent Document 1to carry out two exposures by dipole illuminations in X and Y directionsusing a mask bearing a lattice-like pattern. When this method iscompared with the above method using two masks, the optical contrast issomewhat reduced, but the throughput is improved by the use of a singlemask. As described in Non-Patent Document 1, the method involves formingX-direction lines in a first photoresist film by X-direction dipoleillumination using a mask bearing a lattice-like pattern, insolubilizingthe X-direction lines by light irradiation, coating a second photoresistfilm thereon, and forming Y-direction lines by Y-direction dipoleillumination, thereby forming holes at the interstices between X- andY-direction lines. Although only a single mask is needed, this methodincludes additional steps of insolubilizing the first photoresistpattern between the two exposures, and coating and developing the secondphotoresist film. Then the wafer must be removed from the exposure stagebetween the two exposures, giving rise to the problem of an increasedalignment error. To minimize the alignment error between two exposures,two exposures must be continuously carried out without removing thewafer from the exposure stage. The addition of s-polarized illuminationto dipole illumination provides a further improved contrast and is thuspreferably employed. After two exposures for forming X- and Y-directionlines using a lattice-like mask are performed in an overlapping manner,negative tone development is performed whereupon a hole pattern isformed.

When it is desired to form a hole pattern via a single exposure using alattice-like mask, a quadru-pole illumination or cross-pole illuminationis used. The contrast may be improved by combining it with X-Y polarizedillumination or azimuthally polarized illumination of circularpolarization.

In the hole pattern forming process using the resist composition of theinvention, when two exposures are involved, these exposures are carriedout by changing the illumination and mask for the second exposure fromthose for the first exposure, whereby a fine size pattern can be formedat the highest contrast and to dimensional uniformity. The masks used inthe first and second exposures bear first and second patterns ofintersecting lines whereby a pattern of holes at intersections of linesis formed in the resist film after development. The first and secondlines are preferably at right angles although an angle of intersectionother than 90° may be employed. The first and second lines may have thesame or different size and/or pitch. If a single mask bearing firstlines in one area and second lines in a different area is used, it ispossible to perform first and second exposures continuously. In thiscase, however, the maximum area available for exposure is one half.Notably, the continuous exposures lead to a minimized alignment error.Of course, the single exposure provides a smaller alignment error thanthe two continuous exposures.

When two exposures are performed using a single mask without reducingthe exposure area, the mask pattern may be a lattice-like pattern, a dotpattern, or a combination of a dot pattern and a lattice-like pattern.The use of a lattice-like pattern contributes to the most improved lightcontrast, but has the drawback of a reduced resist sensitivity due to alowering of light intensity. On the other hand, the use of a dot patternsuffers a lowering of light contrast, but provides the merit of animproved resist sensitivity.

Where holes are arrayed in horizontal and vertical directions, theabove-described illumination and mask pattern are used. Where holes arearrayed at a different angle, for example, at an angle of 45°, a mask ofa 45° arrayed pattern is combined with dipole illumination or cross-poleillumination.

Where two exposures are performed, a first exposure by a combination ofdipole illumination with polarized illumination for enhancing thecontrast of X-direction lines is followed by a second exposure by acombination of dipole illumination with polarized illumination forenhancing the contrast of Y-direction lines. Two continuous exposureswith the X- and Y-direction contrasts emphasized through a single maskcan be performed on a currently commercially available scanner.

The method of combining X and Y polarized illuminations with cross-poleillumination using a mask bearing a lattice-like pattern can form a holepattern through a single exposure, despite a slight lowering of lightcontrast as compared with two exposures of dipole illumination. Themethod is estimated to attain a substantial improvement in throughputand avoids the problem of misalignment between two exposures. Using sucha mask and illumination, a hole pattern of the order of 40 nm can beformed at a practically acceptable cost.

On use of a mask bearing a lattice-like pattern, light is fully shieldedat intersections between gratings. A fine hole pattern may be formed byperforming exposure through a mask bearing such a pattern and organicsolvent development entailing positive/negative reversal.

On use of a mask bearing a dot pattern, although the contrast of anoptical image is low as compared with the lattice-like pattern mask, theformation of a hole pattern is possible owing to the presence of blackor light shielded spots.

It is difficult to form a fine hole pattern that holes are randomlyarrayed at varying pitch and position. The super-resolution technologyusing off-axis illumination (such as dipole or cross-pole illumination)in combination with a phase shift mask and polarization is successful inimproving the contrast of dense (or grouped) patterns, but not so thecontrast of isolated patterns.

When the super-resolution technology is applied to repeating densepatterns, the pattern density bias between dense and isolated patterns,known as proximity bias, becomes a problem. As the super-resolutiontechnology used becomes stronger, the resolution of a dense pattern ismore improved, but the resolution of an isolated pattern remainsunchanged. Then the proximity bias is exaggerated. In particular, anincrease of proximity bias in a hole pattern resulting from furtherminiaturization poses a serious problem. One common approach taken tosuppress the proximity bias is by biasing the size of a mask pattern.Since the proximity bias varies with properties of a photoresistcomposition, specifically dissolution contrast and acid diffusion, theproximity bias of a mask varies with the type of photoresistcomposition. For a particular type of photoresist composition, a maskhaving a different proximity bias must be used. This adds to the burdenof mask manufacturing. Then the pack and unpack (PAU) method is proposedin Proc. SPIE Vol. 5753, p 171 (2005), which involves strongsuper-resolution illumination of a first positive resist to resolve adense hole pattern, coating the first positive resist pattern with anegative resist film material in alcohol solvent which does not dissolvethe first positive resist pattern, exposure and development of anunnecessary hole portion to close the corresponding holes, therebyforming both a dense pattern and an isolated pattern. One problem of thePAU method is misalignment between first and second exposures, as theauthors point out in the report. The hole pattern which is not closed bythe second development experiences two developments and thus undergoes asize change, which is another problem.

To form a random pitch hole pattern by organic solvent developmententailing positive/negative reversal, a mask is used in which alattice-like pattern is arrayed over the entire surface and the width ofgratings is thickened only where holes are to be formed as described inJP-A 2011-170316, paragraph [0102].

Also useful is a mask in which a lattice-like pattern is arrayed overthe entire surface and thick dots are disposed only where holes are tobe formed.

On use of a mask bearing no lattice-like pattern arrayed, holes aredifficult to form, or even if holes are formed, a variation of mask sizeis largely reflected by a variation of hole size because the opticalimage has a low contrast.

EXAMPLE

Examples of the invention are given below by way of illustration and notby way of limitation. The abbreviation “pbw” is parts by weight. Mestands for methyl. For all polymers, Mw and Mn are determined by GPCversus polystyrene standards using tetrahydrofuran solvent.

Polymerizable monomers of formula (1a) were synthesized in accordancewith the formulation shown below.

Synthesis Example 1-1 Synthesis of Hydroxy-Keto-Ester Compound

Synthesis Example 1-1-1 Synthesis of Hydroxy-Ester Compound

A mixture was obtained by combining 500 g of the diol compound, 290 g ofpyridine and 1,000 ml of acetonitrile. While the mixture was kept atabout 40° C., 377 g of methacrylic anhydride was added dropwise to themixture, which was stirred at 40° C. for 20 hours. The reaction solutionwas ice cooled whereupon an aqueous solution of sodium hydrogencarbonatewas added dropwise to quench the reaction. This was followed by standardaqueous workup. After the solvent was distilled off, the product waspurified by distillation, obtaining 360 g (yield 72%) of a hydroxy-estercompound.

b.p.: 73° C./14 Pa

IR (D-ATR): ν=3488, 2961, 2837, 1722, 1638, 1456, 1404, 1377, 1326,1304, 1258, 1149, 1084, 943, 858, 814, 769, 658, 617, 588 cm⁻¹

¹H-NMR (600 MHz in DMSO-d₆): δ=6.03 (1H, s), 5.69 (1H, m), 4.88 (1H, t),4.08 (2H, s), 3.39 (2H, d), 3.14 (6H, s), 1.88 (3H, m) ppm

Synthesis Example 1-1-2 Synthesis of Hydroxy-Keto-Ester Compound

The resulting hydroxy-ester compound, 360 g, was dissolved in 600 ml ofwater, to which 16 g of p-toluene-sulfonic acid monohydrate was added.The solution was stirred at 40° C. for 4 hours. The reaction solutionwas ice cooled whereupon 8 g of sodium hydrogencarbonate was added toquench the reaction. This was followed by standard aqueous workup. Thesolvent was distilled off, obtaining 279 g of a hydroxy-keto-estercompound. The hydroxy-keto-ester compound was obtained quantitativelyand had a high purity sufficient to eliminate further purification. Asseen from the above formula, the hydroxy-keto-ester compound was anequilibrium mixture of monomer and dimer (white solid).

IR (D-ATR, equilibrium mixture of monomer and dimer): ν=3327, 2948,1716, 1636, 1451, 1411, 1386, 1327, 1295, 1239, 1170, 1157, 1112, 1097,1075, 1024, 951, 930, 916, 873, 812, 717, 654, 597 cm⁻¹

¹H-NMR (600 MHz in DMSO-d₆, only chemical shifts of monomer are shownbecause complex peaks appear from an equilibrium mixture of monomer anddimer): δ=6.09 (1H, s), 5.74 (1H, m), 5.37 (1H, t), 4.96 (2H, s), 4.14(2H, d), 1.90 (3H, s) ppm

Synthesis Example 1-2 Synthesis of Carbonyl Compound 1

In 150 ml of acetonitrile were dissolved 100 g of the hydroxy-keto-estercompound (equilibrium mixture of monomer and dimer) and 66 g ofmethoxymethyl chloride. To the solution kept at about 40° C., 106 g ofN,N-diisopropylethylamine in 100 ml of acetonitrile was added dropwise.The solution was stirred at 40° C. for 12 hours. The reaction solutionwas ice cooled whereupon an aqueous solution of sodium hydrogencarbonatewas added dropwise to quench the reaction. This was followed by standardaqueous workup. The solvent was distilled off. The product was purifiedby distillation, obtaining 128 g (yield 84%) of Carbonyl compound 1.

b.p.: 76° C./12 Pa

IR (D-ATR): ν=2934, 2893, 2828, 1745, 1722, 1637, 1454, 1411, 1371,1324, 1300, 1153, 1114, 1065, 1034, 945, 813, 653, 581, 560 cm⁻¹

¹H-NMR (600 MHz in DMSO-d₆): δ=6.10 (1H, m), 5.75 (1H, m), 4.95 (2H, s),4.61 (2H, s), 4.28 (2H, s), 3.28 (3H, s), 1.90 (3H, 5) ppm

Synthesis Example 1-3 Synthesis of Monomer 1

A solution of 25.4 g Carbonyl compound 1 in 100 ml toluene was addeddropwise to a solution of a Reformatsky reagent, which had been preparedfrom 32.9 g of zinc powder and 49.0 g of tert-butyl bromoacetate, in 100ml of THF at a temperature below 30° C. The reaction solution wasstirred at room temperature for 1 hour, after which it was ice cooled. Asaturated aqueous solution of ammonium chloride was added dropwisethereto to quench the reaction. This was followed by standard aqueousworkup. The solvent was distilled off. The product was purified bysilica gel column chromatography (4:1 hexane/ethyl acetate), obtaining36.8 g (yield 92%) of Monomer 1 as oily matter.

IR (D-ATR): ν=3472, 2979, 2934, 2888, 2825, 1722, 1638, 1456, 1394,1369, 1321, 1297, 1252, 1217, 1154, 1113, 1047, 986, 948, 920, 845, 814,658, 582 cm⁻¹

¹H-NMR (600 MHz in DMSO-d₆): δ=6.08 (1H, s), 5.68 (1H, m), 5.10 (1H, s),4.55 (2H, s), 4.14 (1H, d), 4.07 (1H, d), 3.49 (2H, m), 3.22 (3H, s),2.41 (2H, m), 1.88 (3H, s), 1.37 (9H, s) ppm

Synthesis Example 1-4 Synthesis of Monomer 2

Monomer 2 was synthesized by the same procedure as Synthesis Example 1-3aside from using a Reformatsky reagent which had been prepared fromethyl bromoacetate instead of tert-butyl bromoacetate. Yield 94%.

IR (D-ATR): ν=3473, 2982, 2934, 2888, 2824, 1722, 1638, 1455, 1405,1373, 1321, 1297, 1153, 1113, 1046, 947, 920, 814, 657, 595, 556 cm⁻¹

¹H-NMR (600 MHz in DMSO-d₆): δ=6.07 (1H, m), 5.68 (1H, m), 5.18 (1H, s),4.55 (2H, s), 4.15 (1H, d), 4.08 (1H, d), 4.02 (2H, q), 3.50 (2H, m),3.21 (3H, s), 2.51 (2H, s), 1.88 (3H, s), 1.16 (3H, t) ppm

Synthesis Example 1-5 Synthesis of Monomer 3

Monomer 3 was synthesized by the same procedure as Synthesis Example 1-3aside from using a Reformatsky reagent which had been prepared from2-adamantyl bromoacetate instead of tert-butyl bromoacetate. Yield 90%.

Synthesis Example 1-6 Synthesis of Monomer 4

Monomer 4 was synthesized by the same procedure as Synthesis Example 1-3aside from using a Reformatsky reagent which had been prepared from1-ethylcyclopentyl bromoacetate instead of tert-butyl bromoacetate.Yield 92%.

Synthesis Example 1-7 Synthesis of Monomer 5

Monomer 5 was synthesized by the same procedure as Synthesis Examples1-2 and 1-3 aside from using Protecting agent 1 instead of chloromethylmethyl ether and Carbonyl compound 2 instead of Carbonyl compound 1.Yield 88%.

Synthesis Example 1-8 Synthesis of Monomer 6

Monomer 6 was synthesized by the same procedure as Synthesis Examples1-2 and 1-3 aside from using Protecting agent 2 instead of chloromethylmethyl ether and Carbonyl compound 3 instead of Carbonyl compound 1.Yield 89%.

Synthesis Example 1-9 Synthesis of Monomer 7

A mixture was obtained by combining 31.8 g of Monomer 1, 12.6 g ofpyridine and 100 ml of acetonitrile. While the mixture was kept at about40° C., 15.3 g of acetic anhydride was added dropwise to the mixture,which was stirred at 50° C. for 15 hours. The reaction solution was icecooled whereupon an aqueous solution of sodium hydrogencarbonate wasadded dropwise to quench the reaction. This was followed by standardaqueous workup. After the solvent was distilled off, the product waspurified by distillation, obtaining Monomer 7 in a yield of 81%.

Monomers 1 to 7 obtained in Synthesis Examples have the structuralformulae shown below.

Synthesis Example 2-1 Synthesis of Polymers

Various polymers (Resist Polymers 1 to 6 and Reference Polymers 1 to 3)for use in resist compositions were synthesized by combining suitablemonomers, effecting copolymerization reaction in methyl ethyl ketonesolvent, pouring into methanol for crystallization, repeatedly washingwith hexane, isolation, and drying. The polymers were analyzed by ¹H-NMRto determine their composition and by GPC to determine their Mw anddispersity Mw/Mn.

Resist Polymer 1

Mw=7,900

Mw/Mn=1.82

Resist Polymer 2

Mw=7,100

Mw/Mn=1.79

Resist Polymer 3

Mw=8,300

Mw/Mn=1.88

Resist Polymer 4

Mw=7,800

Mw/Mn=1.81

Resist Polymer 5

Mw=8,300

Mw/Mn=1.86

Resist Polymer 6

Mw=8,800

Mw/Mn=1.82

Reference Polymer 1

Mw=6,900

Mw/Mn=1.75

Reference Polymer 2

Mw=7,100

Mw/Mn=1.76

Reference Polymer 3

Mw=6,800

Mw/Mn=1.77

Preparation of Resist Composition

A resist composition in solution form was prepared by dissolving apolymer (Resist Polymer or Reference Polymer) and components in solventsin accordance with the formulation of Table 1 and filtering through aTeflon® filter with a pore size of 0.2 μm. The components used hereinare identified below.

-   Acid generator: PAG1 and PAG2 of the following structural formulae

Water-repellent Polymer 1

Mw=8,700

Mw/Mn=1.92

Quenchers 1 to 4 of the following structural formulae

Organic Solvent:

PGMEA (propylene glycol monomethyl ether acetate)

CyH (cyclohexanone)

TABLE 1 Acid Basic Organic Polymer generator compound Additive solvent(pbw) (pbw) (pbw) (pbw) (pbw) Resist 1 Resist Polymer 1 PAG1 Quencher 1— PGMEA(2,000) (100) (5.0) (2.00) CyH(500) Resist 2 Resist Polymer 1PAG1 Quencher 2 Water-repellent PGMEA(2,000) (100) (5.0) (2.00) polymer1 (3) CyH(500) Resist 3 Resist Polymer 1 PAG1 Quencher 3 Water-repellentPGMEA 2,000) (100) (4.0) (4.00) polymer 1 (3) CyH(500) Resist 4 ResistPolymer 1 PAG1 Quencher 4 Water-repellent PGMEA(2,000) (100) (4.0)(4.00) polymer 1 (3) CyH(500) Resist 5 Resist Polymer 2 PAG1 Quencher 1Water-repellent PGMEA(2,000) (100) (4.0) (4.00) polymer 1 (3) CyH(500)Resist 6 Resist Polymer 3 PAG1 Quencher 1 Water-repellent PGMEA(2,000)(100) (4.0) (4.00) polymer 1 (3) CyH(500) Resist 7 Resist Polymer 4 PAG1Quencher 1 Water-repellent PGMEA(2,000) (100) (4.0) (4.00) polymer 1 (3)CyH(500) Resist 8 Resist Polymer 5 PAG1 Quencher 1 Water-repellentPGMEA(2,000) (100) (4.0) (4.00) polymer 1 (3) CyH(500) Resist 9 ResistPolymer 6 — Quencher 1 Water-repellent PGMEA(2,000) (100) (4.00) polymer1 (3) CyH(500) Resist 10 Resist Polymer 2 PAG1 Quencher 1Water-repellent PGMEA(2,000) (60) (5.0) (2.00) polymer 1 (3) CyH(500)Reference Polymer 3 (40) Resist 11 Resist Polymer 2 PAG2 Quencher 1Water-repellent PGMEA(2,000) (100) (5.0) (2.00) polymer 1 (3) CyH(500)Comparative Reference Polymer 1 PAG1 Quencher 1 Water-repellentPGMEA(2,000) Resist 1 (100) (5.0) (2.00) polymer 1 (3) CyH(500)Comparative Reference Polymer 2 PAG1 Quencher 1 Water-repellentPGMEA(2,000) Resist 2 (100) (5.0) (2.00) polymer 1 (3) CyH(500)Comparative Reference Polymer 3 PAG1 Quencher 1 Water-repellentPGMEA(2,000) Resist 3 (100) (5.0) (2.00) polymer 1 (3) CyH(500)

Examples and Comparative Examples ArF Lithography Patterning Test

On a substrate (silicon wafer), a spin-on carbon film ODL-50 (Shin-EtsuChemical Co., Ltd.) having a carbon content of 80 wt % was deposited toa thickness of 200 nm and a silicon-containing spin-on hard maskSHB-A940 having a silicon content of 43 wt % was deposited thereon to athickness of 35 nm. On this substrate for trilayer process, the resistcomposition in Table 1 was spin coated, then baked on a hot plate at100° C. for 60 seconds to form a resist film of 100 nm thick.

Using an ArF excimer laser immersion lithography scanner NSR-610C (NikonCorp., NA 1.30, σ0.98/0.78, cross-pole opening 20 deg., azimuthallypolarized illumination), exposure was performed in a varying dosethrough a 6% halftone phase shift mask bearing a lattice-like patternwith a pitch of 90 nm and a line width of 30 nm (on-wafer size). Afterthe exposure, the wafer was baked (PEB) at the temperature shown inTable 2 for 60 seconds and developed. Specifically, butyl acetate wasinjected from a development nozzle while the wafer was spun at 30 rpmfor 3 seconds, which was followed by stationary puddle development for27 seconds. The wafer was rinsed with diisoamyl ether, spin dried, andbaked at 100° C. for 20 seconds to evaporate off the rinse liquid,yielding a negative pattern.

A hole pattern resulted from image reversal by solvent development. Byobservation under a top-down scanning electron microscope (TDSEM)CG-4000 (Hitachi High-Technologies Corp.), the size of 50 holes wasmeasured, from which a size variation 3σ was determined. Thecross-sectional profile of the hole pattern was observed under electronmicroscope S-4300 (Hitachi High-Technologies Corp.). The results areshown in Table 2.

TABLE 2 Hole size PEB variation temp. Dose Pattern 3σ Resist (° C.)(mJ/cm²) Developer profile (nm) Example 1-1 Resist 1 90 28 n-butylacetate perpendicular 2.6 1-2 Resist 2 90 27 n-butyl acetateperpendicular 2.6 1-3 Resist 3 90 27 n-butyl acetate perpendicular 2.31-4 Resist 4 90 27 n-butyl acetate perpendicular 2.5 1-5 Resist 5 90 26n-butyl acetate perpendicular 2.5 1-6 Resist 6 90 27 n-butyl acetateperpendicular 2.4 1-7 Resist 7 90 29 n-butyl acetate perpendicular 2.31-8 Resist 8 95 26 n-butyl acetate perpendicular 2.4 1-9 Resist 9 85 23n-butyl acetate perpendicular 2.5 1-10 Resist 10 95 28 n-butyl acetateperpendicular 2.6 1-11 Resist 11 85 27 n-butyl acetate perpendicular 2.71-12 Resist 3 90 29 2-heptanone perpendicular 2.5 1-13 Resist 3 90 31methyl benzoate perpendicular 2.4 1-14 Resist 3 90 33 ethyl benzoateperpendicular 2.5 Comparative 1-1 Comparative 90 38 n-butyl acetateinversely 5.7 Example Resist 1 tapered 1-2 Comparative 90 39 n-butylacetate inversely 4.9 Resist 2 tapered 1-3 Comparative 95 39 n-butylacetate inversely 3.6 Resist 3 tapered

It is evident that the resist compositions within the scope of theinvention form patterns having dimensional uniformity and aperpendicular profile after organic solvent development.

Etch Resistance Test

On a silicon wafer which had been surface treated inhexamethyldisilazane (HMDS) gas phase at 90° C. for 60 seconds, each ofthe resist solutions in Table 3 was spin-coated and baked (PAB) on a hotplate at 100° C. for 60 seconds, forming a resist film of 100 nm thick.Using an ArF excimer laser scanner (NSR-307E by Nikon Corp., NA 0.85),the entire surface of the wafer was subjected to open-frame exposure.The exposure was in a dose of 50 mJ/cm² so that the PAG might generatesufficient acid to induce deprotection reaction. This was followed bybake (PEB) at 120° C. for 60 seconds for converting the base resin inthe resist film to the deprotected state. The portion where the baseresin is deprotected corresponds to the insoluble region in negativetone development. A reduction of resist film thickness by exposure andPEB was determined and divided by the initial film thickness, with theresult being reported as PEB shrinkage (%).

Further, the resist film was developed for 30 seconds using butylacetate as developer. The thickness of the resist film after developmentwas measured. A dissolution rate (nm/sec) was computed from a differencebetween the film thickness after PEB and the film thickness afterdevelopment.

The results are shown in Table 3.

TABLE 3 PEB PEB Dissolution temp. shrinkage rate Resist (° C.) (%)(nm/sec) Example 2-1 Resist 1 90 12 0.12 2-2 Resist 5 90 14 0.15 2-3Resist 11 90 12 0.13 Comparative 2-1 Comparative 95 22 0.35 ExampleResist 3

A lower PEB shrinkage or lower dissolution rate is preferable in that afilm thickness necessary for dry etching is retained, or the initialfilm thickness can be reduced, which is advantageous in terms ofresolution. It is evident from Table 3 that the resist compositionswithin the scope of the invention show a low PEB shrinkage and lowdissolution rate.

EB Lithography Patterning Test

Using a coater/developer system Clean Track Mark 5 (Tokyo ElectronLtd.), the resist composition of Table 4 was spin coated onto a siliconsubstrate (diameter 6 inches, vapor primed with hexamethyldisilazane(HMDS)) and pre-baked on a hot plate at 110° C. for 60 seconds to form aresist film of 100 nm thick. Using a system HL-800D (Hitachi Ltd.) at aHV voltage of 50 kV, the resist film was exposed imagewise to EB in avacuum chamber.

Using Clean Track Mark 5, immediately after the imagewise exposure, theresist film was baked (PEB) on a hot plate at the temperature shown inTable 4 for 60 seconds and puddle developed in the developer shown inTable 4 for 30 seconds. The substrate was rinsed with diisoamyl ether,spin dried, and baked at 100° C. for 20 seconds to evaporate off therinse liquid, yielding a negative pattern.

Sensitivity is the exposure dose (μC/cm²) that provides a 1:1 resolutionof a 100-nm line-and-space pattern. Resolution is a minimum size at theexposure dose. The 100-nm L/S pattern was measured for line widthroughness (LWR) under SEM. The results are shown in Table 4.

TABLE 4 Sensi- PEB tivity temp. (μC/ LWR Resist (° C.) Developer cm²)(nm) Example 3-1 Resist 6 90 n-butyl acetate 59 4.8 3-2 Resist 6 90methyl benzoate 61 4.8 3-3 Resist 6 90 ethyl benzoate 63 5.0 Comparative3-1 Compar- 95 n-butyl acetate 58 8.5 Example ative Resist 3

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention.Therefore, it is intended that the invention not be limited to theparticular embodiment disclosed as the best mode contemplated forcarrying out this invention, but that the invention will include allembodiments falling within the scope of the appended claims.

Japanese Patent Application No. 2013-005982 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A pattern forming process comprising the steps of applying a resistcomposition comprising a polymer adapted to form a lactone ring underthe action of an acid so that the polymer may reduce its solubility inan organic solvent, an optional acid generator, and an organic solventonto a substrate, prebaking the composition to form a resist film,exposing the resist film to high-energy radiation, baking, anddeveloping the exposed film in an organic solvent-based developer toform a negative pattern wherein the unexposed region of resist film isdissolved away and the exposed region of resist film is not dissolved,said polymer comprising recurring units (a1) of the general formula (1):

wherein R¹ is hydrogen or methyl, R² is hydrogen or a straight, branchedor cyclic monovalent hydrocarbon group of 1 to 20 carbon atoms in whicha constituent —CH₂— may be substituted by —O— or —C(═O)—, R³ is an acidlabile group, R⁴ is hydrogen or a straight, branched or cyclicmonovalent hydrocarbon group of 1 to 20 carbon atoms in which aconstituent —CH₂— may be substituted by —O— or —C(═O)—, and a1 is anumber in the range: 0<a1<1.0.
 2. The process of claim 1 wherein saidpolymer further comprises recurring units (a2) of the general formula(2) and recurring units of at least one type selected from recurringunits (b1) to (b4) represented by the general formula (3):

wherein R¹⁰ is hydrogen or methyl, R¹¹ is an acid labile group differentfrom the acid labile group R³ in formula (1), X³ is a single bond,phenylene, naphthylene, or —C(═O)—O—R¹²—, R¹² is a straight, branched orcyclic C₁-C₁₀ alkylene group which may have an ether moiety, estermoiety, lactone ring or hydroxyl moiety, or phenylene or naphthylenegroup, and 0≦a2<1.0,

wherein R¹³ and R¹⁶ each are hydrogen or methyl, R¹⁴ is a straight,branched or cyclic, di- to pentavalent aliphatic hydrocarbon group of 1to 16 carbon atoms which may have an ether or ester moiety, R¹⁵ and R¹⁷each are an acid labile group, R¹⁸ to R²¹ and R²² to R²⁵ are eachindependently hydrogen, cyano, a straight, branched or cyclic C₁-C₆alkyl group, alkoxycarbonyl, or a group having an ether moiety orlactone ring, at least one of R¹⁸ to R²¹ and R²² to R²⁵ has a hydroxylgroup substituted with an acid labile group, m is an integer of 1 to 4,n is 0 or 1, b1, b2, b3 and b4 are numbers in the range: 0≦b1<1.0,0≦b2<1.0, 0≦b3<1.0, 0≦b4<1.0, 0≦b1+b2+b3+b4<1.0, and0<a2+b1+b2+b3+b4<1.0.
 3. The process of claim 1 wherein said polymerfurther comprises recurring units derived from a monomer having anadhesive group selected from the group consisting of hydroxyl, cyano,carbonyl, ester, ether, lactone ring, carboxyl, carboxylic anhydride,sulfonic acid ester, disulfone, and carbonate.
 4. The process of claim 1wherein said polymer further comprises recurring units (d1), (d2) or(d3) represented by the following general formula:

wherein R⁰²⁰, R⁰²⁴, and R⁰²⁸ each are hydrogen or methyl; R⁰²¹ is asingle bond, phenylene, —O—R⁰³³—, or —C(═O)—Y—R⁰³³—, wherein Y is oxygenor NH and R⁰³³ is a straight, branched or cyclic C₁-C₆ alkylene group,alkenylene group or phenylene group, which may contain a carbonyl(—CO—), ester (—COO—), ether (—O—), or hydroxyl moiety; R⁰²², R⁰²³,R⁰²⁵, R⁰²⁶, R⁰²⁷, R⁰²⁹, R⁰³⁰, and R⁰³¹ are each independently astraight, branched or cyclic C₁-C₁₂ alkyl group which may contain acarbonyl, ester or ether moiety, a C₆-C₁₂ aryl group, a C₇-C₂₀ aralkylgroup, or a thiophenyl group; Z¹ is a single bond, methylene, ethylene,phenylene, fluorinated phenylene, —O—R⁰³²—, or —C(═O)—Z²—R⁰³²—, whereinZ² is oxygen or NH, and R⁰³² is a straight, branched or cyclic C₁-C₆alkylene group, alkenylene group or phenylene group, which may contain acarbonyl, ester, ether or hydroxyl moiety; M⁻ is a non-nucleophiliccounter ion; 0≦d1≦0.3, 0≦d2≦0.3, 0≦d3≦0.3, 0<d1+d2+d3≦0.3.
 5. Theprocess of claim 1 wherein the developer comprises at least one organicsolvent selected from the group consisting of 2-octanone, 2-nonanone,2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone,diisobutyl ketone, methylcyclohexanone, acetophenone,methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate,amyl acetate, isoamyl acetate, butenyl acetate, propyl formate, butylformate, isobutyl formate, amyl formate, isoamyl formate, methylvalerate, methyl pentenoate, methyl crotonate, ethyl crotonate, methylpropionate, ethyl propionate, ethyl 3-ethoxypropionate, methyl lactate,ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate, amyllactate, isoamyl lactate, methyl 2-hydroxyisobutyrate, ethyl2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate,benzyl acetate, methyl phenylacetate, benzyl formate, phenylethylformate, methyl 3-phenylpropionate, benzyl propionate, ethylphenylacetate, and 2-phenylethyl acetate.
 6. The process of claim 1wherein the step of exposing the resist film to high-energy radiationincludes KrF excimer laser lithography of 248 nm wavelength, ArF excimerlaser lithography of 193 nm wavelength, EUV lithography of 13.5 nmwavelength or EB lithography.
 7. The process of claim 1 comprising thesteps of applying a resist composition comprising a polymer comprisingrecurring units of formula (1), an optional acid generator, and anorganic solvent onto a substrate, prebaking the composition to form aresist film, forming a protective film thereon, exposing the resist filmto high-energy radiation, baking, and developing the exposed film in anorganic solvent-based developer to form a negative pattern wherein theunexposed region of resist film and the protective film are dissolvedaway and the exposed region of resist film is not dissolved.
 8. Anegative pattern-forming resist composition comprising a polymer and anorganic solvent, said polymer comprising recurring units (a1) of thegeneral formula (1):

wherein R¹ is hydrogen or methyl, R² is hydrogen or a straight, branchedor cyclic monovalent hydrocarbon group of 1 to 20 carbon atoms in whicha constituent —CH₂— may be substituted by —O— or —C(═O)—, R³ is an acidlabile group, R⁴ is hydrogen or a straight, branched or cyclicmonovalent hydrocarbon group of 1 to 20 carbon atoms in which aconstituent —CH₂— may be substituted by —O— or —C(═O)—, and a1 is anumber in the range: 0<a1<1.0, and said polymer being dissolvable in adeveloper selected from the group consisting of 2-octanone, 2-nonanone,2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone,diisobutyl ketone, methylcyclohexanone, acetophenone,methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate,amyl acetate, isoamyl acetate, butenyl acetate, propyl formate, butylformate, isobutyl formate, amyl formate, isoamyl formate, methylvalerate, methyl pentenoate, methyl crotonate, ethyl crotonate, methylpropionate, ethyl propionate, ethyl 3-ethoxypropionate, methyl lactate,ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate, amyllactate, isoamyl lactate, methyl 2-hydroxyisobutyrate, ethyl2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate,benzyl acetate, methyl phenylacetate, benzyl formate, phenylethylformate, methyl 3-phenylpropionate, benzyl propionate, ethylphenylacetate, and 2-phenylethyl acetate.
 9. The resist composition ofclaim 8 wherein said polymer further comprises recurring units (a2) ofthe general formula (2) and recurring units of at least one typeselected from recurring units (b1) to (b4) represented by the generalformula (3):

wherein R¹⁰ is hydrogen or methyl, R¹¹ is an acid labile group differentfrom the acid labile group R³ in formula (1), X³ is a single bond,phenylene, naphthylene, or —C(═O)—O—R¹²—, R¹² is a straight, branched orcyclic C₁-C₁₀ alkylene group which may have an ether moiety, estermoiety, lactone ring or hydroxyl moiety, or phenylene or naphthylenegroup, and 0≦a2<1.0,

wherein R¹³ and R¹⁶ each are hydrogen or methyl, R¹⁴ is a straight,branched or cyclic, di- to pentavalent aliphatic hydrocarbon group of 1to 16 carbon atoms which may have an ether or ester moiety, R¹⁵ and R¹⁷each are an acid labile group, R¹⁸ to R²¹ and R²² to R²⁵ are eachindependently hydrogen, cyano, a straight, branched or cyclic C₁-C₆alkyl group, alkoxycarbonyl, or a group having an ether moiety orlactone ring, at least one of R¹⁸ to R²¹ and R²² to R²⁵ has a hydroxylgroup substituted with an acid labile group, m is an integer of 1 to 4,n is 0 or 1, b1, b2, b3 and b4 are numbers in the range: 0≦b1<1.0,0≦b2<1.0, 0≦b3<1.0, 0≦b4<1.0, 0≦b1+b2+b3+b4<1.0, and0<a2+b1+b2+b3+b4<1.0.
 10. The resist composition of claim 8 wherein saidpolymer further comprises recurring units derived from a monomer havingan adhesive group selected from the group consisting of hydroxyl, cyano,carbonyl, ester, ether, lactone ring, carboxyl, carboxylic anhydride,sulfonic acid ester, disulfone, and carbonate.
 11. The resistcomposition of claim 8 wherein said polymer further comprises recurringunits (d1), (d2) or (d3) represented by the following general formula:

wherein R⁰²⁰, R⁰²⁴, and R⁰²⁸ each are hydrogen or methyl; R⁰²¹ is asingle bond, phenylene, —O—R⁰³³—, or —C(═O)—Y—R⁰³³—, wherein Y is oxygenor NH and R⁰³³ is a straight, branched or cyclic C₁-C₆ alkylene group,alkenylene group or phenylene group, which may contain a carbonyl(—CO—), ester (—COO—), ether (—O—), or hydroxyl moiety; R⁰²², R⁰²³,R⁰²⁵, R⁰²⁶, R⁰²⁷, R⁰²⁹, R⁰³⁰, and R⁰³¹ are each independently astraight, branched or cyclic C₁-C₁₂ alkyl group which may contain acarbonyl, ester or ether moiety, a C₆-C₁₂ aryl group, a C₇-C₂₀ aralkylgroup, or a thiophenyl group; Z¹ is a single bond, methylene, ethylene,phenylene, fluorinated phenylene, —O—R⁰³²—, or —C(═O)—Z²—R⁰³²—, whereinZ² is oxygen or NH, and R⁰³² is a straight, branched or cyclic C₁-C₆alkylene group, alkenylene group or phenylene group, which may contain acarbonyl, ester, ether or hydroxyl moiety; M⁻ is a non-nucleophiliccounter ion; 0≦d1≦0.3, 0≦d2≦0.3, 0≦d3≦0.3, 0<d1+d2+d3≦0.3.
 12. A polymercomprising recurring units (a1) of the general formula (1) and having aweight average molecular weight of 1,000 to 100,000,

wherein R¹ is hydrogen or methyl, R² is hydrogen or a straight, branchedor cyclic monovalent hydrocarbon group of 1 to 20 carbon atoms in whicha constituent —CH₂— may be substituted by —O— or —C(═O)—, R³ is an acidlabile group, R⁴ is hydrogen or a straight, branched or cyclicmonovalent hydrocarbon group of 1 to 20 carbon atoms in which aconstituent —CH₂— may be substituted by —O— or —C(═O)—, and a1 is anumber in the range: 0<a1<1.0.
 13. A monomer having the general formula(1a):

wherein R¹ is hydrogen or methyl, R² is hydrogen or a straight, branchedor cyclic monovalent hydrocarbon group of 1 to 20 carbon atoms in whicha constituent —CH₂— may be substituted by —O— or —C(═O)—, R³ is an acidlabile group, R⁴ is hydrogen or a straight, branched or cyclicmonovalent hydrocarbon group of 1 to 20 carbon atoms in which aconstituent —CH₂— may be substituted by —O— or —C(═O)—.